This category covers hardware related to the various cartridges and ROMs such as:
- REU (Ram Expansion Units)
- Ethernet interfaces
- Expansion Port Expanders
- RS-232 interfaces
- Kernal switching boards
- Parallel cables
- Extreme Cartridge 2 and it's official site (esp. at hardware) may come back into the race once it is available.
- C64 I/O Expander by Mager Valp (maybe not quite what you are looking for but an interesting hack nonetheless)
- EasyFlash 2 is rumored to have support for several freezers.