vim Improved (vi improved improved)/ vim Everywhere
I don’t think I need to get too much into why vim is great. You can read about what other people think all over the internet. I do want to recommend a few things for people that are already in love with vi keybindings, but maybe want vim closer to an IDE (without learning configuration stuff yet) or people that want to be able to use vi in fun places (like IntelliJ, RubyMine, or in the Ruby IRB REPL.)
Instant vim IDE: spf-13 is a GitHub hosted distribution of vim with add-ons and much preconfiguration. It has some nice documentation on GitHub, and it includes nice things like autocomplete (though I’d like to learn soon how to make it better, which I know you can do,) NERDTree for file traversing, CTRL-P for quickly accessing files, ZenCoding (now called emmet.io) for easy markup generation, TPope’s surround plugin, etc. Another great thing about it, is it already is set up with Vundle, so you can easily browse and install packages.
Alternative to Eclipse (with vim keybidings): Please anything but Eclipse! JetBrains has a much better IDE (my bias, again) called IntelliJ IDEA. Get the IdeaVIM plugin by going to Preferences > Plugins. You’ll want to change the shortcuts to go into and out of IdeaVIM mode, because they conflict with some shortcut for refactoring. I’m just using the Community version of it now (which is free,) but even though it is normally somewhat expensive, they randomly have fire sales (usually on holidays) where you can buy it for cheap. I’ve been using another, very similar, IDE by JetBrains just for Ruby, and it actually responds to mouse clicks, keystrokes, etc., in contrast to Eclipse (maybe I’ll learn to stop hating Eclipse someday. Please, somebody tell me how to enjoy using it).
vim in a Ruby REPL: Check out the interactive_editor gem. It’s very handy!