I have been using the Arachno Ruby IDE for several weeks now and recommend it.

It has just about everything you’d expect in a graphical IDE, with the possible exceptions of “Intellisense”-style completion (which would be difficult if not impossible to do for a dynamic language like Ruby), source control integration, and complete documentation. (It’s still under very active development, so documentation is a work in progress.)

It has (of course) syntax coloring, automatical indentation, graphical debugging (including Rails app), and class and gem browsing. Even though it’s essentially a beta, it’s quite fast and has been very stable in my experience. [Update: Having said that, I should note that Arachno just crashed. The fact that it launches quickly and has autosave made this less of a calamity than it might otherwise be.]

It launches quickly and never bogs down or pauses for garbage collection, unlike say, IntelliJ. IntelliJ is a wonderful Java development environment but also a real resource hog.

Like all good IDEs, it has fast search and file choosing, with incremental filtering as you type file or class names. (The default keybindings are vaguely Emacs-ish, which in my book is a good thing. E.g. Ctrl-P Ctrl-F to pick a file, or Ctrl-P Ctrl-O pick an open buffer.)

The cost is only $29 for personal use, or $89 for the professional edition, with free upgrades for 32 months. I’ll definitely purchase a copy as soon as my trial runs out.