Thanks to the wonder that is stumble upon I have been coming across more and more fun but distracting online games. Give them a try at your peril, they are a major time sink.
Virus 2 - Simple board like game where you have to make your virus take over the whole board.
Tetris - The Daddy of them all. Needs no introduction.
Puki - Kill a rampaging horde of what appears to be toddlers, using a laser gun. Strangely reminiscent of a "philosophical/hypothetical" question raised by CMB, "how many 5 year olds could you kill, if they kept coming at you and you had to?".
Matter - Simple block arranging pattern matching game. Think along the lines of the challenges on the Krypton Factor, if you know what that is. My best so far is 1080 (Update: 1131) points.
Dots - Connect dots to make squares, make more squares than the computer.