An open source (Free Object-Oriented License, FO2L) compression program. It's different from gzip and bzip2 in that it's a lossy compression program. The advanced, unpatented algorithm (though Unisys claimed there was an infringement) can reduce the file size to 0%. It formerly used PLACeBO (Lessiss-Moore implementation), but now needs support functions...
According to Slashdot, it has been fairly portable - most people have succeeded in compiling the program on their platform, whatever that be. (So far, only GNU Hello has achieved that...)
First release: April 1, 2001. (The only damn funny thing I've found during the whole day, and it isn't even that much fun. Darn.)