Icinga (a fork of Nagios)
PRTG Network Monitor - Windows GUI implementation of MRTG's functionality (limited freeware version available).
Darkstat - realtime network statistics. It also offers bandwidth graphs for an interface, as well as traffic to/from specific IP addresses.
RRDtool - Reimplementation of MRTG's graphing and logging features
Cacti - A similar tool using RRDtool
Observium - A heavily automated platform for network graphing using RRDtool
Netflow - Netflow is another option for bandwidth usage analysis. Netflow is a standard means of traffic accounting supported by many routers and firewalls. You need a Netflow collector running on a host inside your network to collect the data. pfSense can export Netflow data to the collector using the pfflowd package, or softflowd.
ntop - If you need even more detail than that, you might need the ntop package. It will even track where connections were made by local PCs, and how much bandwidth was used on individual connections.
vnstat - is another bandwidth monitoring tool available to install as a package. See the Vnstat doc for more information.
sysmon - A network tool designed for high performance and accurate monitoring.
pftop - is a small, curses-based utility for real-time display of active states and rule statistics for pf, the packet filter (for OpenBSD).
Note: Then press the following: 7, Shift +R (Capital R) , s, 1
7 is speed screen
Shift R is sort by RATE
s, 1 is setting refresh to 1 second.
h for help.
q for quit.
bandwidthd - tracks usage of TCP/IP network subnets and builds HTML files with graphs to display network utilization. Charts are built by individual IP.
- systat -ifstat 1
The fork of nagios to icinga is a good thing, much in the same way as quagga was a great fork of zebra.