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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.