Frank Buss is working on an universal C64 cartridge. You can upload files from a PC (via USB) to the cartridge and store up to 31 cartridges (each 64 kB) on the cartridge. It can emulate any cartridge type except for cartridges with DMA access. On his web page you can find more pictures, schematics and information.

Demonstration video of the Universal C64 Cartridge.