Wake Up Brain! is a great tool that can help students Brainstorm ideas for essays, projects, presentations, etc. It is a good way to get students' creative juices flowing before writing. It can also help students when they get stuck on a writing topic.
BrainHQ from Posit Science features online exercises proven to sharpen cognitive skills in 60+ science studies. Think faster, focus better, and remember more with BrainHQ. It's free to start!
MindCipher is a site filled with riddles, brain teasers, and logic puzzles on subjects ranging from math to science. An additional feature is the difficulty level of each puzzle. Moreover, puzzles can be sorted by "most popular" and by difficulty level.