Developer Jon Lund Steffensen is working on an open source clone of Settlers 1, the first in the long running series.
If you're interested, take a look at the code and see what you can do, maybe submit code to Jon and help get it fully working.
Some of the older games were wonderful and the likes of 0Ad, OpenTTD (and many more) are just wonderful open source renditions or improvements on existing but mostly unavailable games. BTW, Settlers 2 is available to buy from gog.com.
- Link to Linux C Games Code Library