RubyMine is amazing for large Rails projects. I can jump between methods in different modules, classes even across gems (a simple command-click on the method, class or module name). You can quickly traverse source trees without the cumbersome grep/find that you'd do in a basic text editor.

Running specific tests is also a snap; just place your cursor in the desired spec and command-shift-r (built in support for spork).

I'm not in any affiliated with JetBrains (swear); just a satisfied customer.