Comparison of several editors: http://geniisoft.com/showcase.nsf/WebEditors

 TinyMCE
Tiny text editor for tiny minds. Scratch that, it is awesome now.
Install guide can be found here .

FCKeditor
Doesn't seem to like IIS. When we try to create a page with a text editor in it, it throws a page cannot be found (for the area with the editor). Not sure if that is because of some relative path error, or because IIS hates us.
We found a ticket on their support page with someone having a similar problem. The ticket was unresolved.
Works fine in Apache.