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Nearly a decade after the first clunky offerings appeared, online tax-filing sites have become almost as sophisticated as their desktop counterparts. They update automatically to reflect changes in the tax laws, and they enable you to store, retrieve, and print your return from any connected computer.

Concerned about security? All these sites say they keep your data on secure servers in data centers with physical and software safeguards. But dialup users might want to stick with shrink-wrapped solutions: The process slows down considerably at 56 kilobits per second. (Read our reviews of top desktop tax software.)

I tried out the five most heavily promoted programs targeting mainstream tax filers -- CompleteTax, TaxAct Ultimate, TaxBrain, TaxCut Online Premium, and TurboTax Online Premier Investments.

Overall, I found TurboTax the best for filers willing to pay a premium for detailed help. TaxAct remains the low-price leader, while TaxCut holds the middle ground.

All five services participate in the Internal Revenue Service's Free File program that offers free electronic filing if your adjusted gross income is $52,000 or less.

To test these services, I enlisted the help of my dad, a CPA-certified pro who was initially skeptical of Web-based tax preparation. We invented data for a composite family -- a married couple filing jointly, with a freelance business, a kid in college, and a live-in widowed mother-in-law.

We discovered that it pays to shop around: Costs range from $16 to more than $100 for a single federal and state return (with desktop software, you can create and print multiple returns for free). The services also varied widely in their ability to import data from personal finance software or last year's return.

Both of us came away generally impressed with Internet tax filing. But we found that not all of the services are savvy enough to detect user error.

View the Tax Sites chart from the April 2007 issue of PC World magazine.