[StGaensler]

Place this rewrite rule in an .htaccess-file in the forum root:

RewriteEngine On
RewriteRule ^([^/]+)\.html$ member.php?username=$1

This has only one disadvantage: you can't place any html-file in the forum-root, because the url would be rewritten by this expression.

@Don Corleone
Take this:

RewriteEngine on
RewriteRule ^([^/.]+)$ member.php?username=$1

Depending on your configuration it could be that you need following line before the rewrite rules:

Options FollowSymLinks