Commodore 64 extension module that includes the 65816 processor.
65816 is compatible with C64's very own 6510, but runs at 20 megahertz, which is kind of more than 1 megahertz. =)
This expansion (that I regrettably don't have, grrrr) allows C64 to do kind of amazing things, like decoding and showing JPEGs in IFLI mode, and playing Amiga MOD files - not to even mention that vector graphics-using things become a lot faster if optimized...