Arx Libertatis 0.1 is here, after one year of development (The code was open sourced in January 2011). This is the first stable release of the GPL-licensed engine for Arx Fatalis. You still need to own the game assets to play, but it can be purchased on GoG for $5.
You can check the releases notes/changelog here, which contains the best bugfix ever :
Improved level loading speed by limiting the progress bar update rate.
If you have never played Arx Fatalis before, it's time to give it a try !