For example, you can press ctrl+a to go to the beginning of the line and ctrl+e to go to the end of the line. The "Meta" modifier can be accessed via alt+ctrl, so you can alt+ctrl+f to go forward a word, or alt+ctrl+b to go back a word. Any motion can be combined with shift to select the text.
These are especially useful with the laptop keyboards. Bonus points if you convert your caps lock key into an additional ctrl key: http://mkaz.com/archives/86/disable-caps-lock-on-mac-os-x/
He provides a custom DefaultKeyBinding.dict which adds more emacs keybindings to Cocoa text controls. I've been using it for years:
I also bind H to ^ and L to $ with
bind -m vi 'H: vi-first-print'
bind -m vi 'L: end-of-line'
(not OS X specific, but related)