Necessary for object recolors to show up in the game. Everyone should have this file in their downloads folder.

The tool that allows for package editing. This is the big dog, the program that opens the sims package files, allows you to modify content and then repackages the content creating new files. It has many features and is used in almost all object and mesh creation.

Maxis's tool for creating new walls, floors and textures.

Inexpensive 3D Program for creating new meshed objects, hair and body. Milkshape is not a completely free program, but compared to some of the other programs out there, it costs next to nothing to get started. There are also many plugins for modding other games.

Powerful, free Photo editing tool.

If you're ready to purchase a commercial product, many creators love Corel Paint Shop Pro. Here is the link to the free trials page at Corel, so you can test it out.

Here is the information page at Adobe for the positively awesome, hugely expensive Photoshop. The new versions are outrageously pricey, however you can sometimes catch the older versions for sale greatly reduced.