A few months ago, one of the teachers that I worked with approached me about finding a website that gives online tests. We investigated a few different options, but always came up with the same problem: either it was free and you could only take a limited number of tests or you had to pay.
Gnowledge allows you to create tests online for your students to take. You can generate tests online with or without a timer feature. It gives you the opportunity to analyze test results or use tests already created. The website says that as of 2012, there are over 2,000 tests available for users. Creating a test is very simple and just requires a log-in. I haven't had the chance to explore it too much, but tell me what you think!