InMon Traffic Server does sFlow (RFC3176), NetFlow, and XRMON.
Demonstration is at www.inmon.com/its.
Foundry implements XRMON in the JetCore ASIC. Every port can become an
XRMON/sFlow probe. You can set the sampling rate to sample off every
port in real time with sFlow. SNMP data is piggybacked/pushed (not
polled) with sFlow data. Unlike NetFlow, you can implement this on
every port without a performance impact. InMon Traffic Server becomes a
real time tool in this instance. I think this is better than NetFlow
(whitepapers from statisticians convinced me), but we do NetFlow too.
Regardless, I believe it is reasonably priced and you can download a 30
day eval to see if it meets your NetFlow needs. It installs on RedHat
and it doesn't take long to get it up and running.