log data passed in outgoing connections

Hi!
I run a dedicated FreeBSD server with root access.
Some programs working on this server
establish outgoing connections to other web servers.
I need to see what data exactly they send out and probably log this info.
Preferrably some free software that would transparently log this stuff
without making problems to the server or breaking connections.
Or maybe I can just configure some firewall or smth...
Any advice welcome!