Here are some good network monitoring tools:
Log collection and analysis - splunk
Graphs - cacti, PRTG*
Inventory management - netdot
Change management - rancid
Netflow - nfsen
* PRTG is licensed based on number of sensors. E.g. taking SNMP and NetFlow data from a single device means utilizing 2 sensors.
Performance testing tools:
Spirent, Ixia, etc. are hardware traffic generating tools with measurement functionality
Jperf, Iperf, Apache JMeter, PCATTCP, Ostinato are freeware packet generating software tools
