Carmack has released id's previous engines, they're pretty interesting to read, and folks are still maintaining them. The technology ioquake3[1] is 10 years old now, but well-maintained as an open-source project. Darkplaces[2] is a heavily-updated Quake I engine that maintains backwards compatibility with Q1 while using fairly modern game programming techniques. Google ported Darkplaces to NaCl a while ago, and it has renderers in OpenGL, Direct X, and OpenGL ES now.
[1] http://svn.icculus.org/*checkout*/quake3/trunk/README [2] http://git.xonotic.org/?p=xonotic/darkplaces.git;a=summary