You can't load new games into it "as-is", but you
can connect a PS/2 keyboard, an extra joystick, a PSU and a C64 floppy drive to it.

No special components required, just solder the wires and start hacking.
This should get you started:
http://thedarkside.ath.cx/dtv/ports.htmlI've been told there's a version 2 of the joystick around the corner, featuring a USB port for transferring programs to the D64. But that's just something I've heard, consider it a rumour.