We use two different editors at my job. XStandard and Telerik. XStandard is great if you have major concerns about the content being XHTML formatted.
There are a couple of drawbacks however, the editor runs on the client (like Flash) and must be installed by the user upon their first visit. This isn't a major concern if you're running an intranet app, but if it's available to the public then it could be kind of a pain.
The other main drawback is you can't do CSS overlays with XStandard, the editor will sit on top. Not cool if you are trying to do a fancy site.
Telerik is great in the areas that Xstandard is weak, however, it's reformatting of pasted content into XHTML is spotty.
Both of those options are fairly pricey. For free? I'd probably go with FCKEditor.
Hope that helps.