PAQ

PAQ is actually a whole series of different compression programs and their associated file formats, designed to achieve maximum compression (for which the various versions have won some awards) at the possible expense of being slow and taking a lot of memory. Since compatibility (either forward or backward) among the different PAQ versions was not preserved, there are a wide assortment of incompatible formats associated with this group of programs, with new versions devised to implement improved compression techniques or techniques carefully tailored for particular sorts of data without regard for retaining the ability to compress or decompress the files created by earlier versions.

Since PAQ is open-source, there have been "forks" producing yet more adapted and incompatible versions.