Hawksmont Universe released over 30 sets of brushes including G.I.M.P and Photoshop brushes. They are free to use in non-commercial and commercial designs. These brushes have different themes, e.g. gun brushes, skyline brushes, finger prints brushes and splatters brushes,etc.

Requirements: G.I.M.P or Photoshop
Demo: http://hawksmont.com/blog/
License: License Free