The Insiminator! I would most likely get bored between expansions without it. It just makes the game so much more interesting with all the things you can do with it and so readily at hand.
*edited to add, and since OFB came out I should include most of Squinge's "fix" hacks and the great stuff at MATY. OFB is great but wasn't done as well as it should have been and these guys have solved alot of the not-so-well-thought-out stuff in OFB.