Assuming you've got simple taxes (you don't work for yourself, didn't pull from your retirement, etc. You'll probably know it if you have "complex" taxes) and live in the USA, Free Tax USA will let you file free federal and has the cheapest state filing I've found. They're also a fairly trustworthy company that's not owned by Intuit and their corporate allies, so that's a plus.

EDIT: Adjusted the level of trustworthiness.