UPX (the Ultimate Packer for eXecutables) 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.
UPX aims to be Commercial Quality Freeware.
These details are provided for information only. No information here is legal advice and should not be used as such.