To a math teacher, a worksheet generator can be a lifesaver. Sometimes you have the kid who always finishes everything fast and needs more work. Sometimes you have the kid that needs more or less challenging problems. Or the kid that wants extra practice for a test. Sometimes you forgot to write that test or quiz you were supposed to write the night before...a worksheet generator can help you with all of those problems.
My favorite generator out there is produced by Kuta Software (www.kutasoftware.com). Right now they have four programs (Infinite Pre-Algebra, Infinite Algebra 1, Infinite Algebra 2, and Infinite Geometry) and are anticipating Infinite Calculus in the fall of 2012. All you have to do is choose the topic you want, choose how many of those questions you'd like, and choose the options you prefer (size of numbers, variable letters, use positives/negatives, allow zero as an answer, etc). You can opt for open ended or multiple choice questions! The only downfall, in my opinion, is that the answers to the multiple choice answers seem to be random. I'd like to see the answers calculated with a "common mistake" formula that students would be likely to use. But, on the positive side, you can always go in and change the answer choices if you're willing to spend a little time.
From their website you can download sample worksheets or a trial version of the program that's good for 14 days!