I believe there are, or are supposed to have been, a few games where you could connect two PS2s together using firewire and play multiplayer games. I believe Grand Turismo 3 has this capability.
The accessories I have are the EyeToy camera and the Karaoke Revolution mic, both of which work on your Mac.
You can also hook up a keyboard for certain games for text chatting. I know you could use it Metal Gear Solid 3: clomid Subsistence, for example.
Keyboards, mice, printers, EyeToy, flash drives, headsets, microphones, force feedback steering wheels, charging stuff like your iPod or PSP (PSP if you have the right cable)