Not a plugin, but this might help: Codex Template Tags/the widget.
You can then put a text widget in the new ‘widgetized’ area and use code like this to get a sidebar:
<div style="overflow:auto;height:150px;">
<p>First entry and a really long one to see how it handles a long line</p>
<p>another</p>
<p>and another</p>
<p>still another</p>
<p>and again</p>
<p><yet again</p>
<p>one more time</p>
</div>
at matrox they use the html tag <textarea>
http://www.w3schools.com/TAGS/tag_textarea.asp
another possibility is to use a div with fixed height and overflow-y:auto;