Aleph One is a 2.5D first-person shooter engine that is descended from the codebase of Bungie's Marathon 2. This engine is licensed under the GPL.
It uses OpenGL for rendering and supports network play. The Marathon Series has a relatively small yet loyal community which is hosted by Mariusnet. It also used the SDL library.
Downloads such as maps and physics modifications can be found at http://simplici7y.com