I don't like 7zip, they tend to get fudged if you put em on a CD and it gets scratched a little bit, personally for compression I use gzipped tarballs (.tar.gz), there are many shell scripts out there that ensure maximum compression without file corruption that make it a realy handy archive. It would of course make sense for a game using a Quake (2) engine to use the same format as other quake games, quake 3 is open source now and has been for a while, so .pk3's would actually be plausible to implement, as would .pak's.