Best open source Netflow/sFlow analyzing software

People ask me frequently what software I would recommend for Netflow analysis , especially with security implementations in mind.  I made my choice a long ago and haven't been complaining so far - Nfsen graphical frontend that has Nfdump as its data processing backend . It provides most flexibility, configurability; its filter syntax is very tcpdump-like; graphic front provides just enough of interactivity; the alerts system is just amazing.Moreover it supports not only Netflow but sFlow as well,so all Fortigate appliances with the last OS can be monitored this way.