You can rig without extra tools technically, but it's a royal pain the posterior ;o). The big one is a modeling program. If you just wanna create textures for existing clothes, then you already have all you need - Photoshop. There are so many free modeling programs out there (they really must be able to export to .obj) that you can try them all and pick the one that best suits you. I use Roadkill combined with Wings3D to do my UVs - both are free. Some love Blender. There's also Anim8tor and a few others.

Laurie