A few days ago I found out about codesearch. It's a group of command-line tools to index and search code using regular expressions. It was originally developed by fellow googler Russ Cox. It uses the same algorithms that were behind Google Code Search.

I played with it a little bit and I'm impressed by how fast it is. Also, it can be integrated easily with emacs. However, I think it would be even better with a simple web interface. For example, to show the content of the files and highlight matching expressions. Actually that would be a nice weekend project...