Working on Rails code for a bit has started me on the spiral into obsessively customising my dev environment (I say obsessive as at the last Rails meetup I went to there was
In addition to the Rails bundle, there's probably a bundle for your unit test framework (Shoulda, RSpec), and there's also one for Cucumber (if you lean that way). There may be one for your fixture framework too (I know there's one for Machinist, don't know if there's one for FactoryGirl). If you're into HAML there's a bundle for that too, or the Formtastic bundle if you like to use that for your forms.
The Rails bundle is worth it for the migration snippets and association snippets alone. Also "Partial From Selection" is another killer feature.
Of the topic of Ruby/Rails a bit, there's also a really good jQuery bundle. I bring this up because there's a good chance your Rails project is using jQuery. There's also a Prototype/Scriptalicious bundle if you're going that way.
When I'm using TextMate, I'll use Control-Command-T (brings up the "select bundle item" window), in which I can type part of the command and TextMate will filter the list until it finds the one that matches. This is great because I can never remember obscure keyboard commands, nor triggers that I only use a few times a week.