At AnAppADay, Dana Hanna mentioned a search engine for developers named Krugle.

I followed the link, did a basic search and I must say it looks really good. You can refine your search based on language, on project and some other criteria. Impressive!
On the other and we have the recently launched Google Code.

Code.google.com is a site for external developers interested in Google-related development.
It's hard to compare the two sites. Both are search engines but Google's focus is narrowed down to their own software development. In the presentation of the search results Krugle scores again.
Of course this is only a first impression of these sites but for now Krugle is my first choice.
UPDATE:
Google not only searches it's own code base no it can actually search throug all public sources with their "other" search engine Google Code Search.

Dana Hanna aka the Software Jedi is writing one application a day for a period of one month. He writes;
"May the world benefit from the purposeful destruction of my personal life."
You can follow the results of his work at AnAppADay. Only four days left to complete his quest.