I just added the comments preview plugin which adds a button next to the submit button for "Preview". The comment appears where it would eventually end up, but is clearly indicated as a preview. You can get it at http://www.ilfilosofo.com/blog/comments-preview/.

For those that display Google AdSense ads and have the Gregarious plugin installed, you can make the ads more relevant by wrapping the post contents in some tags. There are a couple of plugins that do it, but they also enclose the display content of plugins like Gregarious which makes the ads irrelevant again. You can edit the post badge code in Gregarious to add the tags before and after the <%CONTENT%> part like this:

<!– google_ad_section_start –>
<%CONTENT%>
<!– google_ad_section_end –>

or this:

<!– google_ad_section_start –><%CONTENT%><!– google_ad_section_end –>

In this way, only the articles themselves are targeted.

(Update 2008-04-16: Both plugins have long since been removed.)