Many of the other posts mention programs, which have not been implemented with TDD, and as such do not give an answer to "I'd really like to dig in and see what good TDD/SOLID/etc. looks like." For example, I'm quite positive that the sources of Apache HTTPD Server and Firefox have never even seen TDD. I just browsed a little their sources, and there were only a couple of small files in the test folders, and the production code didn't look like it was written with TDD or by following SOLID either.
For example Fitnesse has been written using TDD by the inventor of SOLID. That should be a good reference of what such code looks like, especially the newer parts of the code (in his book Clean Code, Uncle Bob has some examples of bad code from Fitnesse, which he then cleans up, so not necessarily all code in Fitnesse is clean).
I'll also throw in my own project, Dimdwarf Application Server, which has been written 100% with TDD/BDD and by trying to write as clean code as possible (following SOLID etc.). There are some ugly parts there as well (mostly in task scheduling and garbage collection), but those parts are still under work and they will be cleaned up by the time those features are complete.
