QuizStar is a very useful site that you can use for creating online quizzes and tests. I have currently been using this site for the comprehension questions at the end of each chapter for the different books I use for Novel studies in reading.
I like how the quizzes are organized for the students when they log in.
You can create an account for free, but they do offer a paid version that gives you a few more bells and whistles. I only use the "free" version, as I do with most everything else.
The only thing that I wish were possible with my "free" version would be for students to be able to save their progress through a given quiz. For this to be used, the students need to complete their quiz and submit it, for their answers to be saved.
Sometimes, for students needing more time, I have just copied and pasted the quiz (with their results so far) into Word. On a later date, they can open up and copy and past some answers back into their quiz when they have more time to finish it.
I like this application, because it is web based, and allows anyone with Internet access to work on their quizzes. Students can then complete them at home if they need to or they can even use the HP mini's in my classroom (which are not connected to our school network).