HtmlComponent
I've needed this link ( http://www.nikhilk.net/Entry.aspx?id=11 ) a few times, so I thought I'd give it a little google juice so I could find it easier next time.
The link is to a widget written by Nikhil Kothari, I'm using it in one of my apps currently, and it's really great. Included with a little agility ( .NET Html Agility Pack: How to use malformed HTML just like it was well-formed XML... ), this is a tight little package for editing html in Windows Forms.