There were actually loads of 'Fast Load' cartridges for the
C64, they were 8k
ROMs containing some nifty software that patched the C64 operating system's load and save routines.
The slow part (and it was slow) of disk loading on the C64 was primarily the process of transferring data serially (one bit at a time) down the crappy cable that connected the C64 to the drive, so fast loaders (both cartridge based and pure software ones) used to reprogram the 1541 disk drive's CPU so that it sent the data as fast as it possibly could (using both of the two available wires simultaneously rather than just one, and without any other delays).
The receiving software at the C64 end obviously had to be rewritten too, and this leads us to why the screen was almost always blanked when fast loaders were running: The C64's internal video hardware would frequently interrupt the processor to fetch data from memory to display on the screen (a process known as screen DMA), which would mess with the delicate timing loops that were written to fetch the data from the disk drive as fast as possible. Blanking the screen let the CPU run at full (and consistant) speed, which made the whole thing a damn sight easier to program.
So, software fast loaders worked fairly well, but not as well as the hardware parallel kits, which required you to open up your C64 and your drive, but which transferred data a byte at a time and were blazingly fast. The most popular of these kits was called Dolphin Dos, but I had one called the Disk Demon, made by Datel, and it was bastard quick..
As an extra bonus, all these software upgrades used to include handy extra functions (such as displaying disk directories rapidly, disk copying programs, etc).
Oh, and the rumour isn't true. They didn't hurt your computer at all. And you can shut up with that slander to about the Game Genies too - I designed 'em, and they definitely didn't hurt your console, so nyaaah ;-)