There are three great game engines out there that I know of, however
licensing may be an issue for you (I don't know what you're looking to do
with them, what license you're looking to release a game/games under).
Crystal Space: crystal.sourceforge.net
All three are open-source. I believe Cube and Crystal Space require that
you license any derivative work under the GNU General Public License.
Irrlicht is released under the zlib license, so games based on it do not
have to be released as open-source games (in fact, you don't even have to
mention that the games use the Irrlicht engine).