If you don’t have hundreds to spend on the newest art program like Photoshop or Illustrator there are free alternatives. Many of these are based on the professional programs. They include some of the same features such as brushes, filters, layers, and more.
Gimp: http://www.gimp.org
Gimp is a very popular and well-supported program with similar features to Photoshop. It can be used to retouch photos or create original art. It has many of the same features including: Layers, Channels, Masks, Filters and Effects, Pallets, and Brush Dynamics. You can find many custom brushes on the web and here at Deviant Art.
ArtWeaver: http://www.art