I am looking for a program to make a 3d world that you can navigate. Can you recommend any?

Blender?
www.blender3d.org

i took a look at that but it looks like you just make 3d pictures and you can't actually go into it as if it were like a game level.

The only thing I know of is Mojo World, which is for entire planets. Landscape wise, there is Bryce. Unfortunately, neither is for linux. Those are the only 2 that I know of, and I am not sure either can be used for games.

You can do it with blender but youd need to learn a fair bit about it and camera's, lighting etc.
If you created your structures etc. on the 3d plane, set up cameras and rendered the scene from each camera. I guess it just depends how you want to be able to navigate through the world.

OR

Ive used 3DS MAX with the Quake 3 Tempest plugin (WINDOZE). Which will generate an actual world that is playable in quake3. There is a free version of 3DSMAX called gmax which is often used by modmakers for quake3, half life etc. gmax is windoze though, it can be found at Discreet's Website
Im sure the Half Life SDK or Quake 3 SDK would be only a google away

Blender was first commercial andthen people aroudn teh world bought it and gpl'd it as far as I can tell.

The game-develop-part has been gpl'd also recently. The author released it now under the gpl I believe.

So maybe worth looking at...

http://www.blender3d.com/
http://www.blender3d.com/cms/Stand-alones.162.0.html
http://www.elysiun.com/support/browse.php