Some sites do not really classify as an online resource, but could still be of interest...
Goblin Bet has pixel graphics versions on D&D monsters battling each other, and you can bet on the winner:
Goblin Bet has pixel graphics versions on D&D monsters battling each other, and you can bet on the winner:
http://goblin.bet/This is the legendary (and highly illegal) goblin arena.
Strange beasts will fight to the death, while you bet gold on the winner. Pick wisely.