Ettercap is a network sniffer/interceptor/logger for ethernet LANs. It supports active and passive dissection of many protocols (even ciphered ones, like SSH and HTTPS). Data injection in an established connection and filtering on the fly is also possible, keeping the connection synchronized. Many sniffing modes were implemented to give you a powerful and complete sniffing suite. Plugins are supported. It has the ability to check whether you are in a switched LAN or not, and to use OS fingerprints (active or passive) to let you know the geometry of the LAN.
TAP provides packet reception and transmission for user-space programs. It can be viewed as a simple Ethernet device, which instead of receiving packets from a physical medium, receives them from a user-space program and instead of sending packets via physical media, writes them to the user-space program.
Ethernet Frame Diverter is mainly used for setting up a bridge with Squid running on it and transparently caching the Web traffic between the LAN and the router. "Transparent" means that no reconfiguration of the router or the clients is required. It also means that, in case of trouble, the machine can be replaced by a simple crossover RJ45 patch cord.
EtherApe is a GNOME/pcap-based etherman, interman, and "tcpman" clone. It displays network activity graphically. Active hosts are shown as circles of varying size, and traffic among them is shown as lines of varying width. It supports Ethernet, FDDI, Token Ring, ISDN, PPP, WLAN, and SLIP transports. It knows about VLANs, IP, IPv6, TCP, UDP, FTP, and a number of other protocols. Additional statistics windows will let you concentrate on protocols or nodes.