This plugin allows you to alert on down hosts and the breaching of values in your graphs.

Weathermap is a network visualisation tool, to take data you already have and show you an overview of your network in map form.
Support is built in for RRD, MRTG (RRD and old log-format), and tab-delimited text files. Other sources are via plugins or external scripts.

Cacti's Web Services monitoring plugin. This plugin allows you to add Web Site service monitoring to Cacti. You simply add service checks, the desired service check URL, and the expected response from it's interface, as well as any escallation required if a service check fails. The plugin records statistics about the connection to the website, it's response, and can alert when the status changes.

Spine is a supplemental poller for Cacti that makes use of pthreads to achieve
excellent performance.