UPX (Ultimate Packer for eXecutables) is a portable command-line utility for decompression/compression of executable files and can be used for multiple executable formats.
You have to access to significant compression ratios that will permit distribution size reductions that rival other similar tools as well as quick decompression times. There is no noticeable memory overhead due to UPXs in-place decompression. It provides you with the ability to list, test and unpack your executables. Additionally, you are given checksums for both the compressed and uncompressed file, this is maintained internally.
UPX is fully extendable allowing the addition of new executable formats or new compression due to the class layout.
If you are uncomfortable operating through the command line, don't fear there is Free UPX which offers GUI.
Download contains both 32 and 64 Bit.