EasyFlash is a 1 MByte Flash memory card for the expansion port. It supports following cartridge formats:
…and can be programmed directly from the C64. There is a dedicated tool chain available to achieve this. After flashing (via EasyProg) it can be used as simple game cartridge or even store a multitude of onefilers or crt-games with selection menu (EasyLoader).
Furthermore it has some extra features such as 256 Bytes of additional RAM in i/o space and some optional EAPI with basic functions to read and write(!) to the flash. (more info on http://skoe.de/easyflash and the Development site)