A 6-level (well, a few more, but who's counting) map containing tutorials on making a slew of cool effects in Marathon. These range from trivial (more realistic smoke contrails via a single change in the physics model entry) to seriously complex (swinging doors, breaking windows, exploding walls). You even get tasty bonuses like a real laser sight and a working flashlight (this is too cool for words). It's all described in easy-to-follow language, and shows off some of Chris's amazing Killing Time artwork. Absolutely worth a download, for all scenario makers. Upgrade adds curved archways, far better breaking windows, and better weapons modification information, as well as a prettier package.
Two levels, the first sort of a concept level. Very interesting architecture, but the switched doors get old after a while. If you're building sweeping spaces, download this map... you might get some very good ideas.