Centreon have their own fork of Nagios (Centreon Engine) [1] and a pretty nice UI (Centreon) that beats the pants off of the crusty old Nagios CGI interface [2]. We've used the Centreon front end (but keeping Nagios as the underlying monitoring engine) for about three years now and have been very happy with it.

[1]: http://www.centreon.com/Content-Download/download-centreon-e...

[2]: http://www.centreon.com/Content-Download/donwload-centreon