Before exams or even small quizzes, I recommend a number of students to use the Quizlet website. It is great for quickly absorbing and checking your previous knowledge and correcting any wrong answers you have. I think it's great for students who want a relatively more interactive and diverse way of learning and revising for exams.
While not strictly academic, 'The Dialectizer' is amusing. Serving absolutely no purpose, this website can take any sort of web page or typed information and instantly translate it into a dialect of your choice. It's just fun to have students mess around with, especially when learning about dialects.
Choices of dialect include: Elmer Fudd, cockney, jive, redneck, Swedish chef, and pig latin.
The "Free Rice" site is a good one, as suggested in the post above. One of the most interesting websites I've ever been to is one showing the scale of the objects and celestial bodies in the universe in a zoom-in/zoom-out format, going from quarks to galaxies.
Here's a link: http://htwins.net/scale/
I love the "Free Rice" site. Here, one can learn new vocabulary while donating rice to developing and needy countries. What a great way to learn and give back at the same time.
Are you looking for websites that have to do with some particular field? If you would like to, please clarify so we can have a better chance of telling you about sites that will actually help you. If you are just wondering about sites of any sort, one of my favorites is sporcle.com. You can waste a lot of time there taking quizzes of various sorts.