Brilliant Blocks

In this Tetris-like online puzzle game, you have to clear as many blocks as possible by matching rows of three colors or similar numbers, matching them up vertically, horizontally, or diagonally. Earn bonus time by thinking quickly!

http://www.learn4good.com/games/online/brilliant_blocks.htm

TwinCol

Games for your brain, indeed... and also addictive! Follow the squares of colors and patterns, and match them up by clicking on pairs. Seems easy enough, right? Try getting up to level 6, where the squares are moving across the panel at a fast pace, and intersecting with each other. Careful; some colors may look alike, but will have different patterns.

http://www.gamesforthebrain.com/game/twincol/

PLANET SUDOKU

There's a brand new puzzle everyday and online here for all Sudoku lovers. You can also play a random puzzle from the archives. Click "Play" and then select your level of difficulty, Easy, Medium, or Hard.
http://members.learningplanet.com/act/sudoku/free.asp