<script src="http://whizzywing.com/whizzywing.js"></script>in the head section of your HTML and any element with a class="WYSIWYG" will be turned into a rich text editor. It's not limited to textareas. It's rich in features, too: correct me if I'm wrong but no other editor this small provides table editing. That's if there is another editor this small. With just 9 items on the toolbar you can do most of the formatting that you could do with CKEditor or TinyMCE but using a fraction of the bandwidth and without the installation headaches. It is easy to pick up and is more than powerful enough to use in a CMS.
There has been a lot work under the bonnet to keep the generated HTML sane, no matter what browser you are using. There won't be a load of browser specific classes or deprecated font tags. Hit enter after a paragraph or a heading and you will get another paragraph - not a div, or orphaned text separated off with extra break tags. That means there won't be problems if you come back and edit the same text in a different browser.
The code is licenced under MIT and LGPL. You can download it, or create your own fork, at github. Try it at Whizzywing.