I found this neat mob, which instantrespawns.
Location: http://img839.imageshack.us/img839/9278/222ae.png
North West corner of the Gate of the Setting side.
The mob you're killing is the Krik'thik Needler - NPC - World of Warcraft
I also found out, that if you pull multiple mobs inside of the circle where he stands, and kill them on almost the same time, they both instantspawn at the same location.
They literally re-spawn faster than you can kill them, as mobs also flood in from the sides.
Gonna test later to see how much exp per hour you can get from farming this mob.