Ain't Fate A Funny Thing
So here I am lamenting the lack of free (well, open source, but free is good enough) single player FPS games, and up pops this on Freshmeat; Excalibur: Morgana's Revenge:
EMR 3.0 is a unique scenario based upon the Aleph One (Marathon) engine. It is a first-person action adventure game, featuring an epic and in-depth story line.
The Aleph One engine is hardly cutting edge (originally powering the Marathon trilogy of games, circa 1996-1998) as you can tell from the EMR screenshots, but it takes a long time to produce a fully fledged single player FPS. EMR boasts 42 SP levels and a decent storyline. The game works on all 3 major platforms (Lin, Win, Mac) and is downloadable from their Sourceforge project page. I don't have time to verify how open source this game is but the source code is listed amongst the downloads.