UPX

UPX, the Ultimate Packer for eXecutables, is a free and open source executable packer supporting a number of file formats from different operating systems.

UCL has been designed to be simple enough that a decompressor can be implemented in just a few hundred bytes of code. UCL requires no additional memory to be allocated for decompression, a considerable advantage that means that a UPX packed executable usually requires no additional memory.

Website
upx.sourceforge.net