Comment Exult and Pentagram (Score 1) 381
I think one of the best examples of this topic has to be the Exile project, which has successfully created a cross-platform, open source, modern-day engine to play Origin Software's classic Ultima VII and Ultima VII:Serpent Isle games.
In a similar vein is the Pentagram project, which aims to make a similar engine (and repeat the same daunting reverse-engineering task) for Ultima VIII.
If you are an Ultima fan, and *haven't* heard of these projects, go and download them right now.