What lives in this nest?

Looks like a hornets/wasps nest to me! If it is watch out doesn't take much for them to swarm!
I'd stay as far away from that sucker as I could and watch it for a while see what is coming and going from there.
If its hornets I'd get some of that long- shot bee and wasp killer and nuke the little buggers but thats just my opinion.
 
Definitely some type of paper wasp, get some spray and nail it after dark!

Kristina
 
AquariaCentral.com