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 is capable of intercepting traffic on a network segment, capturing passwords, and conducting man-in-the-middle attacks against a number of common protocols.

It supports active and passive dissection of many protocols (even ciphered ones) and includes many feature for network and host analysis.

It is free software, licensed under the terms of the 
GNU General Public License.

Ettercap-NG requires libnet. A SlackBuild for this is available at 
SlackBuilds.org.
