Free UPX is a graphical interface for UPX, the "ultimate executable packer", and can help you compress or decompress various executable file types (EXE, DLL, OCX, CPL etc).
The package will often compress executables by more than standard archiving software, and the results are transparent. Compress an EXE, it keeps the same extension as before, and decompresses itself every time it's launched with no sign it was ever packed.
UPX is aimed at developers who need to reduce the size of their distribution (NirSoft uses it to keep file sizes low, for example). But you could use it to shrink your favourite applications, maybe to squeeze more onto a USB key.
You could also use Free UPX to decompress packed executables. That's handy if you're trying to analyze an EXE with something like PeStudio to find out what it does.
Whatever you're doing, Free UPX is simple to operate. Drag and drop some EXEs to see information about them, check if any are packed already, then compress or decompress them in a couple of clicks.
Beware, though: by default Free UPX rewrites the original file with its packed/ unpacked version. To change this, enter a custom output file name, or just work with a copy of the source file.
That's not all. Packing the wrong files may cause problems with some applications, if it doesn't break them entirely, so be sure to test your exe's carefully.
Added support for the latest version of the UPX: 3.93.
Free UPX is simple and straightforward packer/ unpacker which supports many different file types (Windows, Elf, Mac OS). It's not aimed at the average home user, though, so if you don't really know what you're doing, be sure to work with copies of the source files.