Ettercap was born as a sniffer for switched LAN (and obviously even "hubbed" ones), but during the development process it has  gained  more and  more features that have changed it to a powerful and flexible tool for man-in-the-middle attacks.  It supports active and passive dissection  of many protocols (even ciphered ones) and includes many features for network and host analysis (such as OS fingerprint).

Characters injection in an established  connection:  you  can     inject characters to the server (emulating commands) or to the client (emulating replies) maintaining the connection alive !!

Packet filtering/dropping: You can set up a filter script that searches for  a  particular  string  (even  hex)     in  the TCP or UDP payload and replace it with yours or drop the entire packet. The  filtering     engine can  match  any     field of the network protocols and modify whatever you want (see etterfilter(8)).