The .pak file format is extremely simple. It has no file magic. The "header" is just the number of entries it contains. A entry is just a file name (with\as path seperator) and the contens of the file. All integers are encoded as little endian. ...
fezpak: pack, unpack, list and mount FEZ .pak archives psypkg: pack, unpack, list and mount Psychonauts .pkg archives unvpk: extract, list, check and mount Valve .vpk archives u4pak: unpack, list and mount Unreal Engine 4 .pak archives t2fbq: unpack, list and mount Trine 2 .fbq ...