Voice Editing
Megadrive Voice Editor:
http://pessoa.fct.unl.pt/gdc18989/sor/hack/MDV%200.8.zip
http://soronline.net/downloads/MDV.zip
Sprite Editing
GIMP:
http://www.gimp.org/downloads/
Adobe Photoshop (This is not free, but there is a trial):
http://www.adobe.com/cfusion/tdrc/index.cfm?product=photoshop&loc=en_us
Patching Utilities
Segatool:
http://soronline.net/files/segatool.zip
LunarIPS:
http://soronline.net/downloads/lunarips102.zip
Character Editing
Pancake 3 v.1.1:
http://soronline.net/downloads/Pk3v11.zip
SoR2 Spritesheets:
http://soronline.net/downloads/sor2_full_sprites.zip
BKIII & SoR3 Spritesheets:
http://soronline.net/downloads/sor3_full_sprites.zip
Palette Editing
Palettes of Rage v1.3:
http://soronline.net/downloads/PoR13.zip
Hex Editing
Hexposure (Windows 7 or later will not run this, but I'm not sure about Vista, though.):
www.romhacking.net/utilities/350/
HxD:
http://mh-nexus.de/en/downloads.php?product=HxD
SoR3 Font Tables:
http://soronline.net/files/sor3fonttbl.zip
Game Genie/PAR Code Converter(To help find addresses in the ROMs):
http://www.romhacking.net/utilities/23/
Hacking Tools from Sonic Retro
There are tools that should work for any Genesis/Mega Drive game:
http://info.sonicretro.org/Sonic_Hacking_Utilities