I wanted to find a way to use the Home and End keys to go to the beginning and end of a line, as I find that more convenient than using Command-Left Arrow and Command-Right Arrow. Here's how I solved it...
First, install Butler, then create two new "Keystroke" Smart Items -- click the Plus sign, then select Smart Items » Keystrokes from the pop-up menu. Set the first to map Home to Command-Left Arrow, and the second to Command-Right Arrow.
[robg adds: You could use any keyboard macro tool to do this; Butler is probably overkill if you just want to set up these two simple macros. And obviously, you can extend this to reassign any keyboard key to a functionality you prefer, as long as it has an existing OS X keystroke.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20091113104558655