Mach 5 by Access Software was a disk speeder cartridge / disk combo for the Commodore 64 that greatly enhanced the transfer speed of the 1541 floppy drive.  This is the disk image that is included with the cartridge.  On the disk are two machine language monitors that load into $c000 and $8000.  There is also a disk organizer program and last but not least, a version of BASIC that has an additional 4k of programming space.

Here is a link to the manual