ChatSniff is an easy to use program for Linux that monitors, or "sniffs" networks for AIM, ICQ, MSN, Yahoo! and Jabber instant messages.
It requires recent versions of python (>= 2.4), python-gtk2 (>= 2.6), python-glade2 (>= 2.6), and tethereal (>= 0.10.10). python-pyao (for sound) and gksu are optional, but recommended (the deb and rpm packages require them).