bzip2 is a portable, lossless data compressor based on the Burrows-Wheeler transform. It achieves good compression and runs on practically every (32/64-bit) platform in the known universe. The compression engine is also available as a library (libbz2), should you desire a programmatic interface.
LZO is a portable lossless data compression library written in ANSI C. It offers pretty fast compression and very fast decompression. Decompression requires no memory. In addition there are slower compression levels achieving a quite competitive compression ratio while still decompressing at this very high speed.