"Instant messaging is everywhere nowadays, but people who use it
may be surprised to know how trivial it is to listen in on their
private conversations. Snoopers can use tools like tcpdump and
aimsniff to tap into the contents of the messages. But with a
little free software, IMers can be secure in the knowledge their
conversations are, well, secure.
"Gaim is the most popular free instant messaging platform.
Available for most Unix operating systems, Mac OS X, and Microsoft
Windows, Gaim can connect to most IM networks, including AIM,
Yahoo, MSN, and Jabber..."