UPX 3.91 has been released with 64bit support

UPX 3.91 has been released. Changes include

experimental support for Win64/PE

To know more about this release, see the detailed changelog. Please report all problems and bugs in our bug tracker.

UPX achieves an excellent compression ratio and offers very fast decompression. Your executables suffer no memory overhead or other drawbacks for most of the formats supported, because of in-place decompression. UPX strengths in a nutshell:

excellent compression ratio: typically compresses better than WinZip/zip/gzip, use UPX to decrease the size of your distribution!
very fast decompression: ~10 MB/sec on an ancient Pentium 133, ~200 MB/sec on an Athlon XP 2000+.
no memory overhead for your compressed executables because of in-place decompression.
safe: you can list, test and unpack your executables. Also, a checksum of both the compressed and uncompressed file is maintained internally.
universal: UPX can pack a number of executable formats.
portable: UPX is written in portable endian-neutral C++.
extendable: because of the class layout it's very easy to add new executable formats or new compression algorithms.
free: UPX is distributed with full source code under the GNU General Public License v2+, with special exceptions granting the free usage for commercial programs as stated in the UPX License Agreement.

You probably understand now why we call UPX the "Ultimate Packer for eXecutables". UPX aims to be commercial quality free software, based on experience with our previous packers (DJP, lzop, and the NRV library).

Homepage: http://upx.sourceforge.net/