Frozen Bubble [www.frozen-bubble.org]

Man, if my boss wasn't me, I would lose my job playing this game in business hours!

It's open source, and it's as addictive as I remember Tetris being when it first came out. It's actually a very similar concept - kind of like tetris upside down with discs that have gravity fields.

RedHat users will find pain free installation RPM files at this site: http://oandrieu.nerim.net/frozen-bubble/

The main site downloads page includes versions for Windows and MacOS X, but I haven't tried them yet...