For example as a google summer of code project, I think it would be cool to have a plugin for a few development environments like VS.NET, Eclipse, Anjuta, Emacs, GEdit and Vim to have code completion and LXR-like cross referencing that uses web services like google codesearch.
Like for example showing the implementation of a method when hovering over it (in the hint), or simple things like a popup-menu item when right-clicking on it that says: do a google codesearch on this function. Or a full pane in your development environment that in the background loads search results of the function near where your cursor is currently located. And many other such features.
I know this sounds like total overkill. Frankly, as a developer I really really don’t care how much bandwidth, memory and CPU my development environment uses. If that helps me develop applications better and faster, then yes I want it.
Privacy issues aside maybe a team could use statistical data from Google codesearch to get information about what developers have difficulties with, and based on that information develop interesting development tools or write documentation.