ctags has become the de facto standard for simple code indexing tasks. Editors that support ctags, such as Vim and TextMate (with the TmCodeBrowser plugin), can read ctags-compatible tags files to allow you to quickly find information about symbols. You can jump to the location where a symbol is defined, perform autocompletion against names defined in the tags file, and get module and type information for any listed symbol. For example, here's a screenshot of Vim using the output of jsctags to inspect the attr function from jQuery:
Moreover, jsctags has more useful output for developers: it knows about CommonJS modules and the CommonJS packaging standard, and it can automatically detect the module system in use.
There will be more in store for jsctags in the future! Dimitris Vardoulakis is beefing up the parser to support Mozilla-specific extensions and is investigating the possibility of adding type inference. In the near future, we'll be using jsctags in Bespin to provide code completion.
Feel free to leave suggestions in the comments and fork the project on GitHub. Feedback is much appreciated!