GCFScape is a tool for viewing and extracting the files inside the asset archives of Quake, GoldSrc, and Source games. Though its name references Valve's former GCF format, it's capable of reading files out of PAKs, VPKs, WADs, BSPs, and more.
GCFScape is an actively maintained project. The latest version featured on this page is v.1.8.6. If this is out-of-date, you can grab the newest version at Nem's website (though his seems to be down at the moment, here's a mirror) and contact us to update the link.