I can't say that I'm a fan of crossovers, but I do like the idea of shared universes such as Outland is in the same universe as Alien.
The Aliens/Predator thing is pretty decent and I have to confess to having a soft spot for the first movie. Outside of that, I think my only experience of crossovers was Dredd Vs Batman or Dredd Vs Aliens. I didn't think too highly of them.
Warehouse 13 could possibly be the same warehouse that the Ark of the Covenant was stored at the end of Raiders of the Lost Ark. There was also a part of me that thought that Firefly and Killjoys could've been in a shared universe too.
Let me think, Baylor.