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.
UPX is copyrighted software distributed under the terms of the
GNU General Public License, with special exceptions granting
the free usage for commercial programs as stated in the
UPX License Agreement.