CL-ROGUE
Over the last six months I have been porting the 1981 version of Rogue to CL. My intention was to learn the basics of CL; it's a straight port of the game. I wish I had thought of porting something sooner (especially Rogue, which I once ported to PC/GEOS), but for some reason this didn't occur to me until recently. It is a great way to learn a language if you don't have much time. I heartily recommend it (over, say, trying to think of a "pet project" to code up in the new language, which is cool, but requires a substantial investment). With porting, the project is already written and working, and you can pick it up and drop it frequently (if you have only a few hours a week, for example) without losing too much context (the original authors did all the hard work!) In the case of C and CL, porting was a good way to learn all the side-effect-y features of CL that are often glossed over in CL books (which focus more on functional programming).
Having spent most of my career doing C/C++/Unix (and a bit of C#/Windows), it is interesting to see first-hand the alternative road that CL would have taken us down. One cannot compare C and CL directly, because in CL the operating system and language are indivisible. That is probably its biggest strength. By contrast, although C is the "first class" language of Unix, it was not treated especially royally. For example, you cannot use C directly in the command shell. When I was first learning Unix, it was confusing that I couldn't call C functions from the so-called "C Shell".
At first, CL's REPL seems alien. Once you realize it is the same thing as the command shell, all makes sense. For example, I had a little difficulty understanding what ASDF was until I realized it's equivalent to make or ant. Many CL functions and packages correspond to Unix utilities that you'd normally have to run from the command-line. Likewise, the lack of an Eclipse- or Visual Studio-like debugger at first seemed to be a major weakness, until it dawned on me that you're always in the debugger in CL.
Porting C to CL was mostly mechanical and surprisingly easy. What was most intriguing was that some obscure-sounding features of CL proved useful (unfortunately I didn't discover this until late in the game). In particular, SYMBOL-MACROLET and DEFINE-SYMBOL-MACRO helped immensely (or at least they did once I figured out how to use them!)
The port itself is on GitHub.