Sometimes we need to improvise on the go, maybe we are using someone's laptop or desktop and they do not have the latest version of PhotoShop or Illustrator. here are a couple of programs to get the job done in a pinch.
Paint.NET - Windows only photo editing software that supports layers, undo's and many special effects.
GIMP - It is a free open source PhotoShop like image editor with many features and functionality.
Xara Xtreme - A great free application which is very similar to Adobe's crown jewel.
Inkscape - A great free vector based program with capabilities that are very similar to Adobe Illustrator.
Karbon - Free vector application with an easy to use interface and lots of features.
Blender - The best free 3D rendering program out there.
K-3D - Very good 3D modeler and animation application.
Google Sketch up - Leave to google to once again come out with a great free application that gets the job done. This software has many powerful features.
Synfig - A powerful vector-based 2D animation software application.
Let me know if you have any more that you have used and recommend!