Getting comments on your site is one of the most important goals. Longer comments tend to be especially valuable.
However, most comment textareas have a small height, making it difficult to write long comments.
Enter Autogrow
Here’s how it works: When a commenter almost fills the textarea, instead of getting a scrollbar, the textarea grows in height. This means they can see the entire comment while it’s being written, no matter how long it is.
It uses the Growfield plugin for jQuery.
Links: Demo | Plugin News | Author’s Site