Gimp-2
The only free 2D paint program that come close to the commercial image editing software, you must have this program.

www.gimp.org

Blender 3D
Open source is the dream tool for professionals but not easy to use unless you learn and accept the odd user interface, I only use its UV mapping features and may explore the game kit to compile 3D scene.

You can't figure out the functions by common standard because it was meant for the pro from the beginning, if you are not going towards such direction, I wouldn't recommend it.

www.blender.org

trueSpace
Caligari is now a part of Microsoft and  all of a sudden, professional 3D program becomes free which spark the beginning of a new era.

It is an indispensable free 3D application though UV edit feature is still under expectation, however, giving its real-time display quality, I give it two thumbs up and believe it will lead the way to collaborative 3D exchange.

www.caligari.com

Sketchup
Google Earth's primary 3D program for anyone who wish to take part in the grand idea and learn simple 3D architectural geometry easy enough for kids, there is no rendering only stiff looking shapes for real-time display.

On the other hand, if you are developing 3D game, I recommend it along side with
3D Rad that easily export objects & scenes to Direct X format.

http://sketchup.google.com
C4D-CE6
Cinema 4D is the best professional 3D software I have come across in the span of 10 years,  the free CE6 version though with render size restricted to 600x400 is the easiest and highly intuitive program to achieve hassle free 3D experience, I recommend it to ordinary people and multimedia designer.

You can run it without installation just unzip the folder to your PC and enter the serial number at first launch, there is no documentation to this version and no longer supported by
Maxon.net but I will provide a series of simple tutorial to achieve designated purpose in the 3D-ebook templates.

Download:
CINEMA 4D CE6

SeaMonkey
Mozilla SeaMonkey is a html editor to create web page or browser based publication like ebook and multimedia documentation.

I will show you how to create interactive photo album with themed design created in 3D customizable widget and compile them into one exe file with easyebook.

www.mozilla.org

3D-assets.info

Free Software