Google has a new search product focused on source code. It peeks inside tarballs and other recognized formats, allowing you to search the index by regex, license, or language. It’s pretty easy to see how many projects are using a given library (such as feedparser or magpie) and keep inventing new ways to explore software.
You can access the code search engine through a GData Atom feed for easy integration wherever you choose.
I find Google Code Search is easier to use than Koders, and may come in handy when looking for different ways of approaching a particular programming problem or library.