I've installed munin on Debian 6 (Squeeze) out of the basic repositories. Then I wanted to open the Munin graphs, by adding "/munin" to the URL, but it wouldn't open.
In /var/log/apache2/error.log it says that it won't allow access to the munin HTML folder:
[SOLUTION]client denied by server configuration: /var/cache/munin/www
By default, Munin installs a configfile for apache (/etc/apache2/conf.d/munin), which only allows connections from localhost (127.0.0.0/8, ::1). If you want to open the Munin HTML page from other computers in your LAN too, you must modify the "Allow from..." part in Munin's Apache configfile.
In my case, I changed it to "Allow from all", but one could (and possibly should) limit that down to the IP ranges of your LAN.