WavPack lossless audio compression. Note that, when converting .wav to this format and back again, the RIFF header is not necessarily preserved losslessly (though the audio is).
This format was originally created as an alternative to mp3. The idea was to achieve a small file size with better sound quality. The first versions weren’t quite successful, but with the development of AAC, it became possible to store sound with less loss of quality, and with the same file sizes as that of mp3.