This is an excellent tool that you can use to look at pk3s and zip files, and it does its job with supreme excellence. This gem gets the effiles.com award of excellence :D
This is a registry file which lets you work with PK3 files as compressed folders. Works quite well, which is something that can't be said for WinZip or similar programs. Pakscape is specifcally
This amazing util lets you open pk3, and root about but also so lets you convert file formats to some thing you can work with! It does too much stuff to list here Well worth a download. :)