This may be known as I am sure others have figured it out the same as I did but I have yet to meet anyone IN-Game that knows about it.
For melee weapon, go into SFK make you way to Deathstalker Vincent and attack him. He will fall over like he died but if you keep right clicking on him he will keep popping right back up and if you click fast enough he wont even try to fall and this can go on forever and ever right-clicking your skill to 300.
Ranged weapons… Go to Ironforge and enter the tram station Go all the way to the opposite wall from the entrance into Ironforge and start shooting rats. They respawn faster than you can kill them. Its non stop carnage.
I even had an undead priest there trying to grief me by killing the rats but they respawned so fast it didn’t slow me down one bit.
(members of the Horde can do this one too if they want to chance it.)
Anyway if you already knew about this, Awesome, and if you didn’t… Enjoy!