These filing requirement pages will help you decide if you MUST file. Even if you determine that you are not required to file it may be in your interest to file. If the IRS determines that you, for example, made an error on your return due to negligence or ignorance they have three years to question the return. If you do not file that year, there is no time limit for questioning the taxes for that year. Filing starts the clock and lets the IRS know what's happening. Of course, if you are deliberately using fraudulent methods to avoid taxation, this 3 year lookback period does not apply. In that case the IRS can examine your return no matter how long ago you filed.
You might also want to file to get a refund of taxes that were withheld from your paycheck. You might want to file to get certain credits you might be eligible for, eg. the Earned Income Credit.
Before electronic filing, unnecessary filings cost us all money. With efile, IRS processing costs $0.17/return (in 2011). If you can file your return at no cost to yourself (using freeFile, for example), I would recommend filing every year even if you are not required to.
If you are or COULD BE a dependent of another taxpayer, click here to decide whether you MUST file.