ethereal
(as root, in Xterminal) Network analyzer--view the network trafic going through your computer. Included on the RH7.0 "Linux PowerTools" CD. Using ethereal may be unethical in some situations, and unauthorized use in the workplace could be a fireable offence.
tcpdump -i ppp0 -a -x
(as root) Print all the network traffic going through the first over-the-phone interface (ppp0) as ascii and hexadecimal. Probably too much printout. tcpdump is a rather raw tool and it can be useful for building more "customized" tools for listening to/log what you need.
