7-Zip is a file archiver with a high compression ratio.

Zzip, a new powerful compressor under GPL, with very high compression ratio

BiGZiP is a FREE Java based compression utility. It does what other programs do but in a Java Environment, so no matter what your OS is, as long as you can run Java, you can run BiGZiP.

UPX is a free, portable, extendable, high-performance executable packer for several different executable formats. It achieves an excellent compression ratio and offers very fast decompression. Your executables suffer no memory overhead or other drawbacks because of in-place decompression.