In case you don't need a text editor, but some comment boxes are way too small, this Greasemonkey script lets you resize them, by dragging a small icon. Another good option is to use the Resizeable Textarea Firefox extension. This bookmarklet should work in most browsers, but you need to click on the bookmark before resizing a textarea.
Somebody suggested that every comment box should be a Google Docs-like rich text editor, but until popular blogging engines support tables and image uploading, Flickr Rich Edit adds some simple editing options for those who don't want to type HTML tags. The Greasemonkey script works only for Flickr's comment boxes by default, but you can add the options to any textarea by replacing http://*flickr.com/* with * in the list of included pages.