knock - knockd is a port-knock server. It listens to all traffic on an ethernet (or PPP) interface, looking for special knock sequences of port-hits. A client makes these port-hits by sending a TCP (or UDP) packet to a port on the server. This port need not be open -- since knockd listens at the link-layer level, it sees all traffic even if it is destined for a closed port. When the server detects a specific sequence of port-hits, it runs a command defined in its configuration file. This can be used to open up holes in a firewall for quick access.
mrtg - Multi Router Traffic Grapher
nload - Nload is a console application which monitors network traffic and bandwidth usage in real time
pcapsipdump - tool for dumping SIP sessions (+RTP traffic, if available)
softflowd - Softflowd is flow-based network traffic analyser capable of Cisco NetFlow(tm) data export.
tcpdump - tcpdump dumps the traffic on a network
tshark - Terminal based wireshark to dump and analyze network traffic
vnstat - Network traffic monitor with log
vtun - Virtual Tunnels over TCP/IP networks with traffic shaping, compression, and encryption.