For the Sudoku fans there is Jigsawdoku. This is the standard Sudoku puzzle game, but with a twist that makes it easy to play online. The numbers are tiles that you can drag and drop with your mouse.
You can also set some interesting options including different size grids, using letters or symbols instead of numbers. and more.
For those who may not be into Sudoku, you can try your hand at Think Draw. This is a little drawing applet that allows you to use pre-defined objects to create your own artwork. If you create an account you can save your creations and share they in the gallery. Even if you are not into drawing yourself, you should check out the gallery to see some pretty amazing little masterpieces.