I have discovered the graphical diff tool Meld, which is notable for a couple reasons. One is that it understands Subversion, so when I type meld collection_controller.rb it shows me the output of svn diff collection_controller.rb, but graphically. I also like that you can quickly merge differences by clicking the arrows between different sections of code.
Topping off my discoveries is ruby-debug. It's been around for awhile, but has yet (but not much longer) to be a standard part of Rails. It's fast and can drop you into the debugger anywhere you type debugger in your code. From there you can type where to view a stack trace, list to see where you're at in the source file, next to step over, step to step into, and cont to continue and be dropped back into the mongrel output.