The Castle is a rather intriguing looking fantasy FPS where your main weapon is a sword, so combat is generally very short range. Currently this open source game only comes with 3 levels and 5 types of enemy, but is very well documented for prospective contributors. The game engine is a custom one written using Pascal and compiled with FreePascal. The game runs on Linux, BSD, MacOSX, and Windows (LBMW). :-)
Whilst originally created for one of the Pascal Game Development (PGD) annual competitions with the current [realistic] scope as the game focus, the author's long term ambitions make for interesting reading:
Basically the game is intended to be like FPS but with some nice storyline. This also means that it should feel more like RPG (in more-or-less fantasy setting) — large world, many items, weapons, some character stats and levels etc. Also I want to utilize my engine to make levels more "interactive" — some objects on the level are able to move etc. There are many such games, some very old, some quite new. I'm just going to do this once again, in the way I want, and make it really good :)
The game engine behind The Castle looks pretty decent considering it's homebrewed and the author has created a few other games (well, demos really) to show it off - malfunction (where you blow up alien space ships) and let's take a walk (where you, er, walk around) which are both available for LBMW.
There are also a few interesting links from The Castle website. The game uses textures from the Golgotha Collection which is a massive collection all in the public domain (I think).
Going back to the PGD website and there seems to be a lot of information on creating games with Pascal as well as lots of entries in the game competitions - 18 this year - which last 3-4 months. Perhaps a few more open source jewels to be found there. ;-)
There are now Linux and Mac OS X binaries available for Privateer Gemini Gold 1.02 rc1 if you want to try that before they release 1.02 final. I ran into a few problems - related to the Vega Strike engine that PGG uses - getting it to run on my rather recently setup Ubuntu Feisty Fawn laptop.
Still, more cool models for VS continue to be made. I hope they focus on a new VS release after PGG 1.02 because there's just so much change since the old 0.4.3 which was years ago.