Google Code Search received a small update: Google indexes snippets from web pages and individual code files, not only repositories and source code archives. You can also submit links to archives, CVS or Subversion repositories to be included in Google's index.
The ranking algorithms now favor class and function definitions.
A good alternative to Google Code Search is Krugle (read about its history), which has a less comprehensive index and doesn't support regular expressions for queries, but compensates with a great AJAX interface, tabs that allow you to open more search results in the same page, syntax highlighting and an easy way to browse within a project directory. Krugle also has a search engine for tech articles, documentations and books and it powers SourceForge's search engine. One thing I especially like about Krugle is you can search on specific code features such as function calls, function definitions, class definitions and comments.
Another popular search engine for code is Koders, which offers very good enterprise solutions, like integration with IDEs and version control systems. The index is less comprehensive than for Krugle and Google Code Search, and it returns worse results than the other search engines, but, according to Alexa, Koders is more popular than Krugle.
It would be nice to have a source code search engine that has Google's index and powerful queries, combined with Krugle's elegant interface and its attention to details and the business-oriented features from Koders.