In OS X, Apple has built Readline/Emacs-style key bindings into most of it’s apps. I’ll add a link to the complete list of these keybindings. but the best one ever is that Control-A will place the cursor at the beginning of your current sentence. Great if you want to place the line you’ve just typed in quotes or parentheses.
(Use Control-E to go to the end of the current sentence.) This works in Safari, TextEdit, Mail.app and most other applications that are part of the OS. (Of course, it also works on the command line.) Sadly, it does not work in Firefox.