There are several HTML/PHP/Java-based editors that are meant to sit in a web page (served from a web server) for editing content. I wonder if these can be used locally and then the contents copied and pasted? I know of FCKEditor (fckeditor.net), TinyMCE, etc.
However, for most forums or web sites, the editors use the template style sheets and such from the web site, making it that much better integrated than something stand-alone. That's why I use the FCKEditor on several of my web sites.
You might want to try looking at something like a blog editor/submission client system - that might work better locally for you. WordPress comes to mind with it's plugins capability (wordpress.org).