The MRTG (Multi Router Traffic Grapher) is a free open source tool written in perl to monitor the traffic load or bandwidth usage on network links. MRTG generates HTML pages containing PNG images which provide a LIVE visual representation of this traffic.
Zabbix is an enterprise open source monitoring solution for networks and applications. Using zabbix network administrators can monitor and track the status of various switches, routers, servers, network services and other network components.
Here is a comparison of the two :
FeatureMRTGZABBIXOpen source yesyesfreeyesyesWeb based interfacenoyesAuto discoeryno yesSLA, and ITIL KPI metrics on reportingno yes