Here’s another quick-hack Greasemonkey user script I wrote recently.
Stretch-to-fit Textareas is a user script which improves the usability of editable textareas; it causes them to “stretch” vertically to fit their contents, as you type. This behaviour was inspired by that of textareas in FogBugz.
It can be inhibited by turning off the small checkbox to the right of each textarea.
Update: it’s worth noting that this is different from the Resizeable Textareas Firefox extension. Whereas the latter allows the user to resize the textareas by hand, this user script does that action automatically, based on the contents of the field; no manual resize-handle-searching and dragging is required. On the other hand, this user script will only stretch textareas vertically, whereas the extension allows them to be dragged in both dimensions. In fact, the two are complementary — I’m running both, and I suggest you do too ;)
Update 2: here’s a Firefox extension version — Greasemonkey not required!