The two most ubiquitous free WYSIWYG editors are TinyMCE and Freetextbox. TinyMCE is in a ton of stuff including the Umbraco CMS and and the fabulous BlogEngine.net. FreeTextBox is favoured by the likes of Community Server (maybe even your humble Channel 9 editor).

Personally speaking I found free textbox a pain to learn because there was'nt a lot of material around (this may have changed). the forums were information deserts. TinyMCE is very popular and is very well documented. Telerik's documentation is first class, they have tutorials for common scenario's like persisting text to a database (which you can use for the other editors as well).

Of the non-free WYSIWYG editors, Telerik's has all the bells-and-whistles (very loud ones too). Consequently, you will need to copy large files to you webserver to get it to work, where the former two are very lightweight and clean.

I wouldn't fret two much, because at the end of the day these are all just augmented rich text editor controls. A pragmantic view would be to first try TinyMCE (as it's free), there is an excellent tutorial here with it wrapped in Ajax. You soon see that implementing it is a doddle, but if more control is needed then you can pay for it.