We use jQuery for most things in our CMS but for the moment use Cameron Adams' widgEditor as a lightweight RTE. Here we have a fork off of Batiste Bieler's jQuery Lightweight Rich Text Editor that looks like it could be a decent jQuery option in our next release, which is targeted at using nothing but jQuery for behaviour.
Functionality is what you would expect from a lightweight RTE, down to the small footprint of the source code which should be easy to customize for your own work. It has a GPL license and is tested and fully compatible with Firefox, Opera, Safari, IE6, and IE7
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