From one of the more innovative TC developers comes this enormous protoscenario. It's a beta version, so there are stil some rough areas... but on the whole, it's worth every byte. There are 20-someodd levels (some are repeats, in order to allow non-linearity of movement), and everything's changed. There is more cool stuff here than I have space to mention... just download it and play it. The arcade games, especially, are amazing... right up there with Missed Island in originality. Just get it. Beware, though... it's R-rated. There is some nudity... not a lot, but just so you know.
A three-level demo of a gothic masterpiece... it'll make you rethink the way you play Marathon. The art is great (this scenario won Runner Up as Best Use of Anvil in Bungie's map contest), the play is tough, the mood is overpowering. (The download is huge.)