Yep, A searchbox for the actual repos would be a nice feature. Filenames and content please ;)

There's actually a search box in private repositories in the upper right-hand corner that searches within the project. It was turned off for public repositories for performance reasons, I think. Tom's sort of spearheading revamping search in 2011, though, (see: new Gist search), so I wouldn't be surprised if he just improves all the things.

Even though public repositories lack a search box, they are still searchable by adding /search?q=$keyword to the url:

https://github.com/ry/node/search?q=runshell

Are there any greasemonkey gurus out there who could write a script which adds a box for this onto public pages?

I agree. Navigating a source tree with fulltext search is obviously much simpler. I wonder why it's not "obvious" to most developers, habits maybe.

I find it handy. Often I'll want to look at a file, but can't recall the exact name or location. I have a shell script that does a find, skipping certain things, and matching on some parameters.

I can also tell it to open what it finds in vim.

But I probably use the "grep files of a certain type for a certain string and open them in vim" version more often.