In my continuous search for good free animated 3D characters, I look for large repositories of MD2, MD3 and MDL models, that are the most common because they were the formats supported by Quake2, Quake3 and Half-Life. A large one was PolyCount, but in the past years it suffered a redesign and the links to the most interesting models are not available yet.
Anyway, looking carefully, there are other large free collections out there. A very interesting one is InGame.de. There you can find some of the most famous MD3 models, but also some that I could find only there. For example, you can see in the screeshots some nice Gundam-like robots very useful for some space combat game.
As usual, these models are explicitly provided for free when used in Quake3. For other purposes, such as the inclusion in other games, one has to ask for permission to their authors. In my experience, it's not too difficult to obtain the permission, asking kindly and giving proper credits, because every artist wants that his models still survive in the ages and does not fade away when the environment where they lived becomes obsolete. Of course, the model should not be too similar to copyrighted material, as the robots in the screenshots, because in that case one may incur in problems difficult to solve without paying a lot of money.