nProbe is a software based flow solution supporting NetFlow v5, v9, and IPFIX. Using the IPFIX capabilities of nProbe you can export far greater information about the traffic over traditional NetFlow:
HTTP - URLs and network/server latency
DNS - all name resolution activies
SIP,RTP - decodes VoIP traffic and is feature rich with caller-id, codec, dscp values, packet loss, and jitter. Everything you want when troubleshooting or reviewing voice.
nProbe is not free, however it is low cost. The rich detail it kicks out makes it a no-brainer if you have no other flow capable devices on your network.
YAF is Yet Another Flowmeter - This is a free tool supporting v5, v9, and IPFIX. It does not have all of the features that nProbe contains, however this is also a good solution.
If you are not savvy with linux, nProbe offers a Windows version.