Here's a flatfile, threaded forum called SpiderDiscussion. Another one I can't get to work. It's not posting the form info, but I see nothing wrong with it (except poorly done HTML)..
<form action=post.php method=post>
<table>
<tr>
<td><b>Name</b></td>
<td><input type=text name=name size=20></td>
</tr>
<tr>
<td><b>E-mail Address</b></td>
<td><input type=text name=email size=20></td>
</tr>
<tr>
<td><b>Subject</b></td>
<td><input type=text name=subject size=20></td>
</tr>
<tr>
<td><b>Message</b></td>
<td><textarea name=message cols=50 rows=5></textarea></td>
</tr>
</table>
<br>
<input type=submit value=Post>
</form>