Jao's post included a link to a screencast on using emacs and ruby by Marshall Vandegrift. I've been using emacs for 20 years, but I learned a lot of useful little tricks. Vandegrift has a post giving links to the various packages he uses in the screencast. Here's the ones I found interesting.
One of the things I've always liked about TextMate is its signature directory listing on the left hand side and the ease with which that allows you to move between files in a file-based programming language like Ruby (or Java, for that matter). I should have known that emacs could do it. The package is the emacs code browser. Note that there are three packages it depends on. Nothing tricky in the installation, but it takes a little time. Once you get it installed, it's the middle mouse button that selects things in the menu (that, of course, can be changed).
I've used ispell for 15 years, but had never heard of flyspell, a package that's included in emacs and gives you on-the-fly spell checking. Good to know.
Another thing you might find handy (that Vadegrift used in the screencast, but didn't talk about in the post) was electric mode. There are electric modes for various languages, I hadn't looked for one for Ruby. Here's a post from Hyperion Reactor that gives a setup for electric mode for ruby and rails.
Note that I'm not writing all this to convert the dedicated vi users or anyone else. If you've got something that works for you, then good enough. But if you're searching for a editor that's programmable with plenty of headroom, then give emacs a try. There's a steep learning curve, but the view is great from the top (or even half way up)!