Here's a game that combines two of my favorite things - playing Tetris and wasting huge gobs of time.
Giant Tetris is...well...giant! It will take you hours and hours, completely killing your productivity along with your pixels.
The folks at Geekologie played for 5 minutes then let it go on its own for 2 hours and 42 minutes. That's crazy talk. But they also made an amazing tree-looking thing.