How does Stackoverflow implement the resizable textarea?
Is that something they rolled themselves or is it a publicly available component that I can easily attach to textareas on my sites?
I found this question and it doesn't quite do what I want.
That talks more about automatically resizing textareas whereas I want the little grab-area that you can drag up and down.
StackOverflow uses a jQuery plugin to accomplish this: TextAreaResizer.
It's easy enough to verify this - just pull the JS files from the site.
Historical note: when this answer was originally written, WMD and TextAreaResizer were two separate plugins, neither one of which was authored by the SO Dev Team (see also: micahwittman's answer). Also, the JavaScript for the site was quite easy to read... None of these are strictly true anymore, but TextAreaResizer still works just fine.