There's a moderately vulgar-yet-funny website I enjoy called Fark.com. Sometimes, they have photoshop challenges - they offer a picture and challenge readers to photoshop it into something unique.
The other day they had a Star Wars vs. Star Trek photoshopping challenge. Here are a few of the results (click for a bigger version):
I like the tie-in in this next one:
Bonus points if you can spot the third movie reference in this one:
(no link, because that one was already small)
Caution: the following image could be disturbing to some individuals: