Flow-tools - http://www.splintered.net/sw/flow-tools/
Not for the faint of heart. A solid toolset for collection and processing of netflow data. The target audience is the experienced network engineer or tools developer. Written in C/C++ and very robust, this is the sword you want in your scabbard when attacking large flow volumes.
Flavio - http://flavio.sourceforge.net/
A perl/MySQL based netflow collection/analysis kit. Great for small to medium flow volumes, or learning how netflow works.
WANGuard Platform - http://www.andrisoft.com/solutions/traffic-monitoring-and-accounting
WANGuard Platform relies on NetFlow or Port Mirroring / SPAN to provide in-depth network traffic analysis and DDoS detection and mitigation. It can be used to generate traffic graphs and traffic accounting reports per IP, per subnet, per IP Zone or per router interface / switch port.