(Dsniff uses a couple of libaries, such as libpcap, libnet, and libnids)
Squid
http://www.squid-cache.org/
Apache (with OS X)
Network alias not only speeds up web browsing, (with squid),
but lets you replace annoying banner adds with more interesting
graphics, and put Woz on Apple's web page.
All the components are some kind of open source license (see the individual packages), so I'm not responsible for nuclear meltdown, world peace, or Microsoft Armaggedon (R)
(network alias uses dsniff's arspoof to impersonate a router, and squid to ack as a transparent cacheing web proxy)