For those of you who aren't familiar with FUBAR - they are a one-page set of rules from the genius who gave us the Forge Of War game system. They are an extremely clever system that encourages you to get to the battlefield right away, rather than spending hours and hours perfecting your army list or bickering over interpretations of vague special rules. They are absolutely perfect for a pickup game... just toss down two roughly-similar forces, come up with some basic victory conditions, and play the game!
Here's a link to my original review: http://basementgamingbunker.blogspot.com/2010/09/more-fun-rules-part-1-fubar.html And a 15mm battle report: http://basementgamingbunker.blogspot.com/2010/09/fubar-battle-report-clear-desert.html
This game is endlessly modifiable using the core mechanics. On this page you will find a variant that I knocked together to play Space Hulk/Doom: The Boardgame style battles: http://basementgamingbunker.blogspot.com/p/fubar-resources.html
If you haven't given these a chance yet - just try them! You might be amazed how enjoyable it is to play a game like this.