ThatQuiz.org is a great site for tests and quizzes. It's also free.
On the surface it looks very simple - and is. If you dig around in it a little, it is very powerful. You can assign tests over the web and it keeps a gradebook for you. Students can also go to it just to practice things. There are lots of different options for tests and a bank of tests that other teachers have made in a variety of subjects.
We use and really like quia. It started free and now costs about $50/year per teacher. I have a feeling that is what most of the free sites have done, but I could be wrong. It's one tool that does actually help take something "off" the teacher's plate. You know, that plate we keep piling stuff on?