Google has, on the edit profile page, a wonderfully simple UI convention I don’t remember seeing before.

Click on a textbox, and you can start typing. From the moment you start typing, it will add another textbox below it, so you can keep adding items. It doesn’t do that when you put the focus on the textbox though, only once you start typing in it. Wonderfully unobtrusive and effective. A jQuery plugin that does this (getting the details right) would be lovely.