Here you will find many informations about hacking SMW. You can also donwload patches, tools, documents and of course, Super Mario World Hacks!

Romhacking.net
A romhacking site which covers everything related to Romhacking. YOu can also find a lot of tools and hacks for download!

AcmIn's Board
Another Rom-Hacking Forum!

KT's Romhacks
Another site with really great Super Mario Hacks. Check it out. ;)

Zophar's Domain
Here you can find everything about emulation:
A lot of emus, utilies, editors and much more! Check it out!