Commissions for discount brokers have never been lower than they are today. Indeed, some discount brokers have dropped their commissions to zero.

But commissions aren’t the only fees you’ll pay to enjoy the services of a discount broker.

Like banks, most brokers also charge fees, which can range from annual maintenance fees to fees for paper statements and confirmations.

What’s more, it isn’t easy to ascertain what those fees are at most brokerage houses. You’ll have to read the fine print on the broker’s web site or account opening form.

Here are six of the more popular hidden fees to watch out for…

Hidden Fee #1 – Inactivity Fees

It’s no secret that stockbrokers make money when you trade. Indeed, a criticism of many full-service brokers is that they push you to trade frequently to pump up commissions.

Now, discount brokers are trying to push customers to trade actively by charging inactivity fees to customers who don’t trade frequently enough.

WallStreet*e charges a $100 fee for inactive accounts. Interactive Brokers charges a monthly fee that can range from $3 to $10, and E*trade charges $40 a quarter. But you can get that fee waived if you keep enough money in your E*trade brokerage account or have other banking relationships with E*trade.

Buy-and-hold investors, or even those who trade a few times a month, need to look closely at these fees to see when they kick in.

Or better yet: Find a broker that doesn’t penalize you for not trading.

Hidden Fee #2 – Fees to Close Your IRA

Charging fees to close your IRA is one of the more popular ways to keep customers from switching discount brokers.

Fidelity, Bank of America, WallStreet*e, Zecco, TradeKing and Wells Fargo all levy fees to close your IRA. Expect to pay between $30-$75 to transfer your IRA elsewhere.

How does your current discount broker stack up to your expectations? And when is it time to make a change?

Hidden Fee #3 – IRA Custodial Fees

Where some discount brokers charge you to close your IRA account, others, like Zecco, Muriel Siebert and WallStreet*e, charge you to keep your IRA with them — to the tune of $30 a year. Siebert waives the fee for accounts with assets over $10,000.

Seems hardly fair, but clearly, their strategy is to seek frequent traders or those with large accounts.

Hidden Fee #4 – Fees for Paper Statements and Confirmations

If you’re comfortable with online trading, chances are, you’re comfortable with online confirmations and statements.

But what if your Internet service is down, or your email software mistakes your stock confirmation for spam and bounces it?

At firms like TD Ameritrade, Scottrade and TradeKing, you’ll have to pay for the peace of mind of getting paper statements and confirmations.

Each paper statement or confirmation mailed will set you back between $1 and $2.50 at these firms.

Hidden Fee #5 – Annual Maintenance Fees

Beware the investor who has less than $50,000 and doesn’t trade frequently. That must be the watchword of some discount brokers.

Whether to discourage small investors or just to make more money, some discounters are now charging annual maintenance fees for accounts below a certain asset value.

Wells Fargo charges a monthly $25 maintenance fee for accounts below $25,000. Bank of America customers pay $50 twice a year, unless they have a checking account with the bank or assets of at least $50,000.

And even Vanguard Brokerage Services, part of the company known for its uber low-fee mutual funds, levies a $30 annual fee on accounts of less than $100,000.

Hidden Fee #6 – Fees to Transfer Your Account Elsewhere

They may not be giving you the tools or products you want. They may be charging fees or commissions that are too high. They may not even answer the phone when you call.

But discount brokers don’t want to lose your business, and many will tack on a fee just to discourage you from going elsewhere.

Schwab, Wells Fargo, Zecco and TD Ameritrade all charge fees to transfer your account elsewhere, to the tune of $50 to $75 for a full account transfer (less for a partial transfer).

But there is a silver lining: Other brokers have seized on this trend as an opportunity to gain more business. Muriel Seibert, SoGoTrade and Scottrade will all reimburse you up to $100 of fees charged by other brokers when you transfer an account to their firms.