My favorite tool for creating 3d objects is Wings3d http://www.wings3d.com/.

It is free and open source and very easy to learn. It can import and export .obj file as well as other 3d formats. Wings is only a modelling tool. It has no tools for animation, rendering (limited actually) or rigging. It does have UV mapping tools (which seem to rival the bigger more advance applications). Wings3d works in Windows, Macs and the various Unix work alike (with binaries for Windows, Mac and Linux, I think; all others need to compile for their system).

Another 3d modeller is Anim8or, http://anim8or.com/. This one has a higher learning curve but also includes rigging, animation and rendering. It is for Windows only, however. Anim8or also imports and exports .obj format. Anim8or is freeware.

The most popular free and open source 3d modeller is Blender. It is very powerfull but with a very high learning curve. Because of its popularity you should be able to find many tutorials and forums dedicated to Blender.

With a few exceptions, almost any modelling program (paid for or freeware) can import and export in the .obj format.

I hope this helps.