I've been out of the comic loops for years now so I don't know if they still make these or not, but the fact that DC had their own version of Marvel's What If was pretty cool. I had a comic called Speeding Bullets where Superman was Batman and another one where the JLA was in the future with Lex Luthor's brain in Metallo's body, I forgot the title though of it though.
What do you guys think? Any favorite particular stories? Spoilers coming up I guess so beware!