Okay, but really. Aleph one is great, and although the original Marathon games were not indie, Aleph One, the thing that runs Marathon on newer machines, is. I want to warn you though, developers, that currently, developing content for Aleph One is very difficult. Alep One is a great program, it's just that the editors to make Marathon maps all suck, aside from the original, which is hard to get working on modern machines, and still isn't even that great.
Fortunately, a project called Weland (pronounced Wayland) is now in development, and it's by one of the same minds that created Aleph One. Not sure how much longer it will be, but it should be less than a year.
Link to Weland development post: pfhorums.com/index.php?showtopic=4829