Boardgame Geeklist: Free Computer Version of Board and Card Games with Artificial Intelligence Computer Opponents
Check out Puerto Rico and San Juan. There are several versions of both. I have played several of the other games but not on the PC.
This list continues for several pages and includes comments and screenshots.
ADDED - while looking at the other pages I discovered one of my favorite games I can't get enough players for usually - Risk 2210 or Invade Earth as it is called here. There is also a link to the online version of Blokus.