ZLib <img src="/ubbthreads/images/graemlins/smile.gif" alt="" /> I'm using it for years in my own programs. Fast, stable and easy to use. Open Source, but not GPL. BSD-License, so everyone can use it, even in non-open-source-projects. Compresses a 70 MB divinity savegame (just before the final endboss) to 9 MB. On medium compression level that is.
I just love this peace of pure genius ANSI-C code <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
