The jQuery linenumbers plugin turns this...
into this...
By simply doing this...
<script src="jquery-linenumbers.min.js"></script>
neat huh. Line numbers make it easy to track the size of a text area, and look right at home if your dealing with code. It wraps lines intelligently, just because the textarea wraps doesn't mean it's actually a new line. How wonderful it recognizes that.
If you want to get fancy there are some additional options you can pass...
For example:
$('textarea').linenumbers({'20px', 1, 1});
You'll need to give your text area a set width, or the two line numbers won't line up right with the text, this has been tested as a px value, but percents should be ok in any format as long as the col_width is the same. Other than that it should work just fine. Internet Explorer has not been fully tested (due to the lack of a machine that can run it) but should work.