pmacct is a small set of multi-purpose passive network monitoring tool
FlowViewer is a web-based netflow data analysis tool.
free netflow analyzer
Enterprise Network Flow Collector (IPFIX, sFlow, Netflow)
NetFlow iptables module for Linux kernel
TCP and UDP network buffer
Buffers, forwards, and splits flow data including NetFlow and IPFIX
Various TCP/IP servers, clients and network management tools
netflow collecting and processing tools