I don't know why but FreeTextBox has just never cut it for me. It's wonderful and easy to install and it comes as a nice control in a .NET assembly. But it doesn't give you the nicest HTML back, in fact I'd go as far as to say, it mutates GOOD HTML you put in there yourself, this is bad. Luckily there are some other free alternatives such as TinyMCE, which is a nice little editor. The only catch is it's pure javascript and requires a little manual intervention to work with your serverside code.
On a side note: The guys over at CommunityServer probably saw this problem and thought; no matter which editor you pick someone was going to complain, so they implemented the editor control in a Provider model. Clever.
I'm not using CommunityServer so changing my little SUB editor still requires two lines of code change and a recompile, but the flavor of the week for me is Obout's HTML Editor. This little beauty is free for personal use, and commercial use requires only a small registration fee.
This editor not only gives better HTML then FTB but also has some cool things like a Built-in spell checker, quick-formatting and bunches of other little options.
UPDATE(29-Mar-07): I have now had a change to try out FCKEditor, and it does seem to work very smoothly. Also I did notice an option when using it under Mozilla to format 'span' tags rather then the out dated 'font' tags.
It also appears that Obout may have changed their policy since I've made this post because I can no longer see any indication that they want anyone to use the editor for free, they seem to now be pushing downloading it as part of their new suite. So in light of this, FCKEditor may be your best alternative.