One of my favorite features is the flexibility of the Goto Anything dialog (CMD+T or CMD+P) e.g.:

- Goto file: "con pos" -> app/controllers/posts_controller.rb

- Goto line in file: "con pos:100" -> app/controllers/posts_controller.rb line 100

- Goto function in file: "con pos@foo" function foo in app/controllers/posts_controller.rb

Also one that came up recently is CMD+ALT+Q to auto-wrap long comments to your ruler (e.g. 80 columns).