After I studied some of the low-level sniffing tools like DSniff, Wireshark, Ettercap and tcpdump, I noticed that the "webspy" tool of the dsniff package is a really cool thing to have.
But it's really difficult to use, it's just not like the usual network dumping tools!
So I wrote a bash script that runs tcpdump and processes the output - extracting the URLs that are visited through the interface (specified by -i option) during the dump.

To use the script, save it as "URLsniffer", run chmod +x URLsniffer and execute it:
sudo ./URLsniffer
It's important that it's ran as root user, otherwise TCPdump won't function properly.

Here is the script:

#!/bin/bash
#
 
# reset variables
myhost="";
myurl="";
 
tcpdump -s 0 -w - -l $@ | strings |
while read line;
	do 
 
# filter GET requests
	myurl=`echo $line | grep GET | sed -r "s/GET (.*) HTTP.*/\1/"`;
	if [ "$myurl" == "" ]; then myurl=$myoldurl; fi
 
# filter Host headers
	myhost=`echo $line | grep Host | sed -r "s/Host: (.*)/\1/"`;
	if [ "$myhost" == "" ]; then myhost=$myoldhost; fi
 
# once we have a data pair, put them together and echo
	if [ "$myhost" != "" ] 
		then
		url="http://$myhost$myurl";
		echo $url;
		myhost="";
		myurl="";
	fi
 
	myoldurl=$myurl;
	myoldhost=$myhost;
done