Tool and Usage
UPX is tool to pack several executable formats. It is free, portable, and extendable, and well-known.
Why this tool?
UPX is the abbreviation for "Ultimate Packer for eXecutables". It is considered to be a tool with a good compression ratio and fast decompression. It can be used to compress executables, making them smaller, while still having a low overhead of memory due to in-place decompression.
UPX is free and open source software (GPv2+). It has special exceptions for commercial programs (see UPX License Agreement).