For a while I've been reading about the term Software Archeology from different media channels and I find it very valuable. Especially if you are trying to do something interesting with your projects, in those cases is a very good idea to checkout some very well written applications.
If you want to see what Ruby can do, this is the project I recommend. It has several gems in it, including a very complete documentation.
Google has released several open source projects with high quality code and lots of functionality, this set of libraries are great tools when dealing with real-life applications.
A great CDI component that simplifies Spring into fine grained components that are extremely extensible. I use it whenever I can and it's really, but really cool.
This is perhaps the application with more test coverage I've ever seen, a serious attempt to reduce the gap between user acceptance tests and testers.
You might have noticed that most of them are written in Java, I didn't have time to check some other languages, but for sure Scala and Clojure are my next target.