Grand Theft Auto: San Andreas/Cheats

Currently, the most comprehensive guide is at: PS2 Cheats @ GTASanAn.com

There is also a cheats list at GameFAQs.com, but this currently contains many duplicate and non-working codes.

Riters.com hosts a San Andreas cheats wiki.

Codemasters-Project.com has lists of Action Replay Max codes for both PAL and NTSC

EdisonCarter's own site has all the cheats he discovered with his device

All PS2 Button Codes at GTAForums.com. All tested and fully functional.

Action Replay codes at GTAForums.com

"complete" Action Replay codes at GTAForums.com (draws from several sources so possibly more complete than the link above)

Code Conversion Requests at GTAForums.com. If a code is for the "other" region's Action Replay, these users will convert it.

GTA-SanAndreas.com Forums has a list of sites with cheat-device codes.