• GNU Image Manipulation Program (GIMP)
    Don't let the odd name roll your eyes. GIMP is a pretty good replacement for Photoshop, especially if you would like a TWAIN interface for converting paper documents to digital, via a scanner.
    There is a variation-- Gimpshop-- that makes GIMP look and feel like Photoshop, but it appears the development has stalled.

    GIMP site
    http://www.gimp.org

  • Google SketchUp (free version)
    If you've never attempted 3D, SketchUp has a very easy interface. It helps create your ideas with basic forms and perspective. The idea was to use it a easily as drawing. I use it to construct a complex shape from simple forms, then rotate it to the right perspective so I can use it as a drawing model. It isn't as full-featured as the for-sale version.

    SketchUp6
    http://sketchup.google.com
    http://sketchup.google.com/product/gsu.html

  • Blender
    If you have attempted 3D, Blender is a full-blooded quad-modeling, rendering and animating powerhouse. If you learned Maya, Max or some other expensive modeler in school but don't have the bucks to pay for a version, try Blender. Be advised this app is not for the impatient or the faint-hearted. The interface is crammed with cryptic buttons.

    Blender
    http://www.blender.org/

    Blender Nodes tools
    If you can't to do something in Maya or Max, you can render amazing things with Nodes, or write a Python script and attempt the impossible with this! Here following, are URLs to Nodes and the location of a Camstudio video step-by-step of Nodes behaving much like Photoshop's Adjustment Layers. (Yes, it's fairly tech-ish; remember the software and the support is free...)
    http://wiki.blender.org/index.php/Blender_3D:_ActionBook/Nodes
    http://www.blenderuniverse.de/Videotutorials/Microcosmos_Composit.html

    And here is a tutorial for Blender Nodes doing the impossible with green screen images:
    http://www.blendedplanet.com/?Freebie_Tutorials:The_Ultimate_Keyer

    Where to find rapid support when you get stuck in Blender, and very good Beginner video tutorials. Download the free videos, then join the very friendly, helpful forum:
    http://blenderunderground.com/