You can use MultiEx-Commander (has some problems with newer paks and can't repack):
http://multiex.xentax.com/
Or Painfull (you may need MultiEx nevertheless to get the required zlib.dll, or get it directly from
http://www.zlib.net/):
http://forum.xentax.com/viewtopic.php?p=3838 (Note that the "free registration" isn't required, you can download without being registered too).
Or you can use Painkiller itself to do that job:
painkiller.exe -script FS.ExtractPack('../Data/pakfile-to-unpack.pak','../Data/folder-to-unpack-to')
and to repack
painkiller.exe -script FS.CreatePack('../Data/pakfile.pak','../Data/source-path', true)
Note that the path you want to extract to has to exist, else nothing happens and you won't even get a notice.
Last but not least you can use the application I wrote myself to pack and unpack pakfiles (called "Pakman"). Not available in public though, pm me if you're interested.