Spring is upon us, and for many of us that means it’s time to bust out the mop and broom and start cleaning house.

Spring cleaning can also apply to your portfolio. Given the current bear market rally, this may be the ideal time to take a hard look at what stocks should be sold before the market heads into its next leg down.

But not so fast — there are also a number of undervalued stocks that you shouldn’t throw away too quickly. And if you don’t own them yet, when the market turns back down you may want to consider buying them.

During the past year, we’ve witnessed the downward spiral of so many well-respected and trusted corporate giants. But these currently downtrodden companies — and their undervalued stocks — can return to their former glory at some point in the future with a few positive corporate catalysts.

Just when this might take place and to what extent remains an open question, but what doesn’t remain an open question is the ability of corporate America to heal the wounds.

These following 10 companies represent the biggest corporate brand names out there, and you should consider these stocks core long-term holdings.

AT&T (T)

The quintessential American telecommunications giant hung up on investors in 2008, but that doesn’t mean that we can’t call on them in the future. AT&T (T) is still the biggest and most diverse communications provider the world has ever known, and with the growth in their wireless businesses from exclusive service provider deals with Apple and its iPhone and Research In Motion and its BlackBerry Bold, we aren’t ready to cut the line on Ma Bell just yet.

The company’s diverse holdings give it the ability to weather the changing telecommunications tide, so whatever the next big trend in talk might be, you can bet that AT&T will take a prominent seat at the table.

Caterpillar (CAT)

The industrial equipment giant’s brand is synonymous with big, bad earthmoving construction machinery. In fact, it’s doubtful that any major construction project performed in America since the Great Depression hasn’t been performed without the help of a “Cat.”

Given the Obama administration’s focus on shovel-ready infrastructure construction jobs as the answer to what’s ailing our economy, it stands to reason that Caterpillar (CAT) equipment will once again be called upon to help rebuild and remodel America’s crumbling roads, tunnels and bridges. Think about this, every time you hear the words “shovel-ready jobs,” you should think about the makers of the actual shovels. And there is no denying that Caterpillar makes the biggest and best shovels on the planet.

Coca-Cola (KO)

There’s nothing as refreshing as a Coke on a hot summer day. Many Americans share this tasty thought, and while we still love to drink our cola, the recent downtrend in the equity markets has caused Coke’s (KO) share price to fizzle. To be certain, Coca-Cola’s stock hasn’t been hit as bad as some of the other stocks we’re profiling, but the shares have still fallen victim to the pernicious declines affecting nearly every market sector.

One of the factors that may cause investors to fall back in love with Coca-Cola is its international presence. Sure, the global economy faltered in 2008, but the demographics in countries such as China, India and the emerging nations provide Coca-Cola a huge and fertile territory where they can continue to grow their high-margin international business.

Exxon Mobil (XOM)

The world has a big thirst for many things, but perhaps its greatest thirst is for petroleum products.

Crude oil, gasoline, natural gas and various other petroleum-based items are still the figurative and literal grease that makes the world’s engine run smoothly. Without the mechanism for getting gasoline into the world’s tanks, the entire globe would come to a screeching halt.

Exxon Mobil (XOM) is the biggest and best company created for the specific goal of supplying the world with the petroleum-based lather it needs to make the system run. Sure, the price of crude oil is well off its all-time highs, as is the price of gasoline and other related items. But no matter what the price of crude is, and no matter how much slowing occurs in global economic activity, the world will still need to purchase oil — and that means the world will need Exxon Mobil.

General Electric (GE)

The best example of an iconic American industrial giant is General Electric (GE). The company has so many diverse revenue sources that it’s hard to know where to begin. Capital, energy infrastructure, technology infrastructure and entertainment are all part of the GE family. During the past year or so, the company’s capital division has fallen on hard times. But we think that when the pain subsides in the financial sector, GE’s money side will likely regain its health.

On the energy infrastructure side, we know that the Obama plan to put billions of dollars into a new, 21st-century “smart” electric power grid will likely include the biggest and best companies in this sector. GE is unquestionably one of the biggest and best in this field, and their expertise in building out our energy generation system will be a big part of what gets America back on the road toward a brighter future.

Intel (INTC)

The semiconductor giant continues making better, faster and stronger microchips that continue powering the world’s personal computers. But it’s not just PCs that Intel (INTC) chips power. The company’s microprocessors also can be found in enterprise computer servers, industrial equipment, point-of-sale systems, automotive information/entertainment systems, medical equipment — the list goes on and on.

As the biggest chipmaker on Silicon Valley’s block, Intel isn’t going anywhere. The company spends millions in R&D each year to insure that it is the one that creates the next-generation microchip must haves and just announced plans to invest $7 billion in three of its U.S. plants. Sure, Intel shares were caught up in the sell-off of 2008, but when the tide turns, any smart investor will want to seriously consider booting up Intel.

IBM (IBM)

Shares of Big Blue sung the blues in 2008, but that’s no reason to feel sad about the company’s future prospects. The IT giant is still the premier global technology services firm, and that’s not likely to change just because the world’s economy is in a funk. Having weathered numerous economic downturns during its nearly 100-year history (including the Depression years) there is no reason to suspect that IBM (IBM) won’t be able to weather the current economic storm.

The computing landscape is always changing, and companies that adapt to that change are going to be the ones that survive. A big part of successful adaptation is having the capital and the resources at hand to make the necessary adjustments, and no company has proven more adept at adaptation than IBM.

Johnson & Johnson (JNJ)

Consumer health care companies are traditionally great recession plays, because no matter what happens in the economy, people still need their medicines.

For more than 100 years Johnson & Johnson (JNJ) has supplied the world with what it needs to ameliorate its ailments. In addition to marquee consumer brands such as Band-Aid and Tylenol, Johnson & Johnson provides the world with a myriad of prescription drugs, medical devices and medical treatments that keep us all healthy.

The healing power — and the earnings power — of Johnson & Johnson’s products has contributed to the well being of both consumers and investors for a very long time. And while shareholders may justifiably feel ill about the stock over the last several months, the company’s long-term growth prospects will likely serve as a veritable love potion for investors in the years to come.

3M (MMM)

In this tough economic environment, why not choose a company with as many solid revenue-generating divisions as possible? And when it comes to having many sources of revenue, you can find no better example than 3M (MMM).

The diversified giant has its hands in so many market segments that the company is somewhat difficult to classify. Industrial and retail products such as tape are its bread and butter, but it also makes products like filtration systems; products used in the manufacture, repair and maintenance of automobiles, boats and aircraft; medical and surgical supplies; optical film and lens solutions for electronic displays; office supply products; construction and home improvement products, home care devices and even personal protection products.

It’s hard to live life without using a 3M product, and it’s this kind of product ubiquity that makes 3M the kind of stock that always attracts investment capital.

Procter & Gamble (PG)

What single company is most represented by the sheer number of its products in your home right now? We suspect that company is Proctor & Gamble (PG). Beauty products, household care products, health care products, pet products, toothpaste, toilet paper, batteries, laundry detergent — even the diapers on Junior’s behind are likely to be made by Procter & Gamble.

Like the value of the other stocks in our love list, Procter & Gamble shares have been hit by the wider sell off. Corporate earnings are likely to be hurt by the economic slowdown, but companies with diverse revenue streams such as Procter & Gamble will be best able to come out of the current recession with the least amount of collateral damage.

Like we said earlier, nobody knows precisely — or even vaguely — when investors will truly embrace equities again. But what we do know is that when the market starts to head back up, these undervalued stocks will be considered great long-term holdings.