Vi is a text editor with an absolute bastard of a vertical learning curve.
It's so tough, you'll think you're crazy learning it.
And now its lickable on the Mac thanks to the MacVim Mac app!
MacVim adds a slick wrapper of services around the familiar gvim interface including auto-updating with Sparkle, nice integration with Mac copy and paste, Apple-N for new windows, a multiple window interface and transparency (:set trans=10).
Vi is powerful and fast and awesome. It's search and replace regex support is rich and you can run vim commands as scripts. It has syntax highlighting, spell checking and limitless undo. You can navigate without moving your hands from the keyboard. And the keyboard movement controls are used again and again on computers, from unix man pages to Boston.com's The Big Picture.
This little app is so famous, you can run Microsoft Outlook, Word and Visual Studio 2008 with vi editing in-line on a PC using viemu (that app's not free like vim unfortunately).
Thanks to Bram Moolenaar at vim.org for his awesome work and for an application I wouldn't want to live without. Look on vim.org for vim on your operating system.