SMWCentral

SMWCentral is one of the oldest ROM hacking sites around. Started by Kieran Menor and Smallhacker in 2006, SMWC has broadened its range of discussion from just Super Mario World to include Yoshi's Island, Super Mario All-Stars, and Super Mario 64 (among others). There are tons of ROM editing tools on this site as well as great hacks you can play.

Raocow's Talkhaus

If you watch a lot of Let's Plays of romhacks, you've probably heard of raocow. He's been on YouTube for years and has even LP'd some SMBX episodes as well. Talkhaus is the community for everything related to raocow, and they have lots of contests and hack projects that they host.