Codase is an extremely targeted search engine: it only searches and indexes source code. This is a developer's search engine, for sure. I'm not a coder and so I probably didn't get the most out of Codase that I really could have, but from what I was able to see using my very limited programming language, Codase is a great tool for anyone searching for source code help.

Related: Don't forget to check out the rest of the search engines profiled in the 100 Search Engines in 100 Days Table of Contents.