Collectd

Collectd is yet another popular monitoring solution.

Collectd uses a single modular agent dealing with both metrics collection and re-transmission.

Like Munin, collectd uses RRD.
Unlike Munin, Collectd collects data locally (without connecting to a remote host: the agent generates its own RRD locally). Additionally, You may load a plugin (network) allowing both to forward your metrics to some remote host, and to gather metrics from remote peers.

Like Munin, Collectd comes with a list of common plugins.
Unlike Munin, Collectd plugins, are libraries. Creating one is harder. And mostly pointless, as Collectd provides with plugins allowing you to run a script.

Most recently, Collectd introduced a filter plugin based on iptables, allowing you to match metrics using regular expressions and handle them differently.

Collectd comes with a very basic web interface – collection3. You may try it, then would quickly try the first alternative you could get your hands on. A easy-to-setup collectd frontend being SickMuse, using bootstrap API.