From LQWiki

root-tail is a graphical version of the UNIX tail command. What it does is sends a real-time tail output (of any file you choose) to your desktop. Your desktop will show the last few lines and any new entries that are added to the file you specify as they are added.

Tips

If you'd like to monitor your /var/log/messages file in real-time (with a nice, ugly green color ;) and left-justified), you can install root-tail, and run the command:
root-tail -color green -g 80x50+0+100 -shade /var/log/messages &

Here's another, from root-tail --help:

root-tail -g 80x25+100+50 -font fixed /var/log/messages,green /var/log/secure,red,'ALERT'