SmokePing is a deluxe latency measurement tool. It can measure, store and display latency, latency distribution and packet loss. SmokePing uses RRDtool to maintain a longterm data-store and to draw pretty graphs, giving up to the minute information on the state of each network connection.
http://oss.oetiker.ch/smokeping/
We run SmokePing to monitor PersonalTelcoVPN and IPv6 performance: http://donk.personaltelco.net/cgi-bin/smokeping.cgi