2640Rcap is a pair of programs to monitor the bandwidth used by your internet connection. It specifically works with the D-Link 2640R router, but since it relies on their telnet interface, should work with other D-Link/Cisco routers with similar firmware.
A CGI webpage produces a pretty graph of your bandwidth usage over the current month and year. You can use this to monitor how close you are running to any bandwidth cap.
This tool should be of particular interest to TalkTalk customers in the UK as the 2640R is the one issued by TalkTalk.
Requires Python and a CGI compliant webserver.