I'm sure this is VERY well known, but it's always fun to use.
If you're a Night Elf hunter, you can stick your pet on a person, and immediatly after hit Shadowmeld. You will be invisible, and your pet will be eating away at the victim. If they stop and try to kill it, you can let them get down to a low %, then proceed to own them. This is GREAT for BG's. I always hide in the flag room and stick my pet on casters. It's fun watching your pet beat-down on a person, and them not knowing where you are.
Once again, I'm sure its well know, but it's quite fun!