I don't play computer games all that much (heck, I don't play regular games all that much) but sometimes … sometimes there's a game (DICEWARS: A Flash Game) [1] (via kisrael.com [2]) that just sucks me in.
There are no rules presented, but if you've played Risk [3] you should be able to figure out the game play. Oops, gotta go, being attacked on my left flank by green …
[1] http://www.gamedesign.jp/flash/dice/dice.html
[2] http://kisrael.com/viewblog.cgi?date=2006.08.12