For creating patterns, you simply need a graphics program that supports PNG format as a minimum. In order to make 4-color patterns for Sims 3, or to make textures for meshes, your graphics programs will need to support the DDS format and be able to work with alpha channels.
Free graphics programs
GIMP – GNU Image Manipulation Program (DDS and alpha support)
Paint.net (has native support for DDS – needs a Paint.net alpha plugin to import greyscale alphas but this works perfectly)
Mid-price programs
Adobe Photoshop Elements (Photoshop DDS plugins work, but alpha channel support limited to black and white)
Corel Paintshop Pro/Photo (DDS plugins are increasingly hard to come by; alphas have limited support)
Top-end programs
Adobe Photoshop (DDS and full alpha support, including greyscale)
Although top-end programs are often out of reach (and unnecessary) for most people, it is often possible to acquire an older edition (for example: CS2) for around the same price of one of the mid-range programs at places like eBay. Given that most older editions also support alpha channels and DDS formats fully, this can be worth considering.
For more information on working with Sims 3 object textures and graphics programs needed, see: Object Creation Part 2: Object Textures.
