Bah, y'all needs some insane evilness!!!
Summoning underwater has been mentioned, good fun.
Summon into mobs, been mentioned.
Now for some fun stuff.
Say you're summoning someone to Blackrock Mountain. Go right up to the edge just before falling in lava. Ask whoever you're summoning to face in the direction of the lava (using compass on minimap, south, northeast, w/e). Now, tell them to hit the Number Lock key (autorun!) the split second before they hit the accept key. If they are gullible enough and the summon keeps them facing the same direction... lol.
Cast max rank Curse of Recklessness on lowbie mobs and watch them hit like trucks. This is also fun and useful to do on mobs that are being fear kited by another lock, since they will be immune to fear for 2 minutes. Yes, locks, this is what curse of recklessness is for!
Use banish on enemy tagged elementals and demons. Chain banish will make plenty of people give up. However, do NOT do this against hunters, rogues, frost mages, or priests, as they will simply pop Feign Death, Vanish, Ice Block, or Fade (respectively) and dump the mob onto you. A warlock was banishing the demons I was kiting in Winterspring, so when it came out of vanish I ice blocked the mob onto him while he was trying to fight another demon. He almost died, but I was generous and let him live. Fair warning!
That's about all I can think of ATM, fun stuff though!
EDIT: Infernals. That's all I'm gonna say.