Irrlicht is an open source 3d engine.
If you know C++, you can easily use it.
It comes with a package which can be used on Linux,Windows and MacOSX. In other words, you have platform variety.
Using Irrlicht makes making 3d animations and game graphics easy for you.
I used it with GCC compiler and had no difficulties while using. Its documentation is enough for a person who little knows about making graphics.
Irrlicht can be found at: http://irrlicht.sourceforge.net/
You can see screenshots of the projects made with Irrlicht Engine at this address:
And also a part of this project, irrKlang is another powerful tool if you are interested in making a game. It provides the sound engine for your works. It can also be found at the same address.
Enjoy your 3d worlds!