Todays games without amazig 3d graphics seem to be like toy games made by newbie developers. Well this is not my opinion, but after mastering the 2d stuff any serious game developer should jump into the 3d world. But before writing your own 3d engine consider the pros and cons, and maybe have a glance on some stable, existing engines.
For many people the finest open source 3d engine is OGRE. To some people it is not a complete engine, because it lacks typical features of an engine (as physics, networking, etc.), but OGRE definitely has an amzing renderer. Beginners often complain about complexity, ugly documentation, but OGRE has a very nice community. It's suitable for cross-platform development.
Another popular open source 3d engine is Irrlicht. It's is a well designed engine, easy to use, and thanks to the BSD license great for modifications. Some will say it's not as powerful and slow, but this may be perfect for beginners or simpler things. Also Irrlicht has a good community, and is suitable for cross-platform development.
The next one open source 3d engine is Crystal Space. It is the most complete open source 3d engine out there, but is not easy to use. They have a very friendly community, and is also suitable for cross-platform development.
Once crafted by Disney now Panda3D became open source. Well, what to say else than this is another great engine. It is simple to use, and suitable for cross-platform development.
Next one of the open source and cross-platform 3d engine is the Nebula Device. It's a powerful and complex 3d engine, but maybe isn't easy to use because it lacks community and tutorials. It already found its way into commercial game development.
The last of the open source 3d engines I review is OpenSceneGraph. It's also a well designed engine.
Well of course there are many more 3d engines. In fact it seems like there are as many as game programmers. Many of them are open source, but some developers say, they are not suitable for the development of commercial games. For sure there are some commercial engines available, as Torque or some other, but I'm not sure in which way they are superior - since I'm using Unix-like systems I'm not really gaming, and that's before GPU was established.