What that is great!! I believe they made one for the N64, the Zip Disk Drive I believe.
There were a few for the N64. There was one called the Dr. V64, which backed up to CD and the better one (which is the one you're thinking of) was the Mr. Backup Z64. That thing was awesome; I had one and sold it to a member here once I accumulated all of the real N64 games that I wanted.
There are backup systems for nearly every cartridge based console that I can think of (except the Neo Geo). Makes playing some of the more obscure games much easier
EDIT: Someone will probably mention that there was one for the Neo Geo, the Multi Game Doctor. But it SUCKED and didn't work correctly.