Thanks to an IRS program, just about anyone can file a tax return online for free. Here are the details.

Filing a tax return is about as fun as a migraine headache, but now there's a pain reliever: Thanks to an IRS program that started in 2003, now all U.S. taxpayers can file online, free of charge.

Why the gift? Online filing (which automatically does the math that many filers get wrong) significantly cuts down on errors, saving the IRS time and money. And for taxpayers, it has the added advantage of offering refunds within 10 days. (With paper returns, that typically takes six to eight weeks.)

So here's how free filing works. The IRS has partnered with 20 online tax-software companies, giving each of them the flexibility to determine who is eligible to use their free services. Last year, most tax companies used different criteria to restrict eligibility, including income, age or state of residence. But when some of these providers opened their free services to everyone, competition heated up and this year even tax giants H&R Block and Intuit's TurboTax offer free filing to absolutely everyone.

So what's the catch? First, to use these services for free, taxpayers must access them through the IRS Free Filing Web site. If you go directly to the company's site, you'll end up paying the regular filing fee (usually about $20 or so). To make sure you're not going to be hit with that fee, go through the IRS Web site every time you log on to your account.

Also, if your taxes are fairly complicated, you might want to shell out the extra cash for a paid service: Those include additional guidance that may not be available with the free filing options. For example, with TurboTax's Basic product ($19.95) you'll get help figuring out the cost basis of shares you've sold during the tax year that is not available with its free file, according to a company spokesman.

Finally, don't wait until the last minute to e-file: These services may get slower as all other procrastinators rush to the Web sites. And be sure to read all the fine print carefully. Chances are you'll be pitched additional fee-based services (like audit protection or refund-anticipation loans) that you may not need. And if you want to file your state return along with your federal one, prepare to pay up because most preparers charge for state returns.