The Armament Project
Started in 2005, The Armament Project was a royalty-free repository for super-high-resolution gun and weapon models, intended as a resource/reference for weapon modellers. This was in an attempt to improve the quality of 3d weapon art used in games and game mods, and on a personal note it was also a great exercise in improving my skills at 3d work.
Unfortunately The Armament Project never really took off as a resource for modellers, however producing the models was extremely enjoyable and several made it to release. These can be seen below in each of their individual project pages.
Each model is still provided royalty-free for non-commercial use.