These are custom models for Quake 3. Installation is simple.

To Install:
Download desired model and place in desired folder. After downloading, Go into Program Files and find Quake 3 Arena. Go into your baseq3 folder and place the model file in it. It should work instantly by selecting it from the "Players" menu in the game. Note: Internet Players must have model installed on their own computer to see yours, but I have the most common ones here for download so you should have no problem being seen. You will appear as the Sarge model if they do not have the player model, but you will still see yourself as the custom one.