Pucrunch is "An Optimizing Hybrid LZ77 RLE Data Compression Program"
It is a compression system that has been implemented on your average stock Commodore 64, and it decompresses reasonably fast. Compression, however, is written in C, to run it in "bigger" machines for getting efficient compression.
Pucrunch was written by Pasi "Albert" Ojala, and the current implementation is freeware. It's being used in some freeware games (such as Metal Warrior III).
The pucrunch home page (with long details on How It Works and a sample implementation): http://www.cs.tut.fi/~albert/Dev/pucrunch/