Emucenter
This is a good M.U.G.E.N. and emulation page

Mugen 4ever
Here you can find a lot of characters, stages, ad-ons for M.U.G.E.N.

The Underdogs
If you are looking for old computer games, here you can find the best abandonware.

Mame.dk
If a rom works in M.A.M.E. , it can be found here.

Testp U.N.I.T.E.D forum
It was the more visited forum about Mugen, but now it's closed.