Ettercap is a suite for man in the middle attacks on LAN. It features sniffing of live connections, content filtering on the fly and many other interesting tricks.
It supports active and passive dissection of many protocols (even ciphered ones) and includes many feature for network and host analysis.

• Please note ettercap 0.6.x is deprecated; please upgrade to 0.7.
• Binaries are not officially provided or supported - you have to compile it yourself (which can be tricky under Windows) or find a third-party binary provider. The developers are unlikely to be swayed on this.
• Before installation, please ensure you have the correct versions of the above listed libraries. For the best experience you should have them all. Out-of-date or missing libraries are the most common reason for failure.
• You need to read the documentation in the tgz before and after running ./configure; make; make install.
• The command-line switches and the etter.conf have changed from 0.6.x to 0.7. You should read and edit the etter.conf to suit.
• For HTTPS support you need to uncomment the right 'redir' command in etter.conf.
• In short, read the docs, read the man pages, read the FAQ, read the forums, and then ask questions. ;)

• You need to select a user interface (no default) using -T for Text only, -C for the Ncurses based GUI, or -G for the nice GTK2 interface (thanks Daten...).