The Resizeable Textarea Firefox extension turns the mouse into a diagonal pointer when it hovers over the edge of a textarea, and lets you click and drag the field to a larger or smaller size. A must-have folks who type into web page text areas often (bloggers, webmail users, forum posters, any web app user), my most-wished-for feature is that the extension would save a size you specify on a per textarea basis. Free, Firefox required.
Screencast: See Resizeable Textarea in action
Download Resizeable Textarea [Extensions Mirror]