code browsing: I want less than an IDE, more than grep

A question for the pros: I want to dig into the Rails source code to understand How Things Work. What tools do people use for interactive browsing?

In particular, I'd like to be able to point at a method name (or package, or class) in the source code, see a list places where that's defined, and jump to the source code where it's defined.

I'm fluent in emacs, and that would be my first choice: are there any packages that do this?

But I'll switch to textmate if you convince me that's the only way to go. (I tried it on my old Mac and had some problems with it...)