Back in the day I was rather partial to DU Meter for monitoring network traffic on my Windows Server/WinRoute Pro based gateway box. These day's I'm running Linux (Mint currently) and have been looking for an equivalent pretty GUI to monitor network traffic and other things.
As ever the open source community has come up with something which is not only brilliant and eminently configurable, but also has a suitably quirky name: conky.Debian/Ubuntu/Mint users can grab it using the wonderful apt-get facility (other 'nix distros can get it from the Conky home page).
sudo apt-get install conky
Once installed, modify conky by editing its config file using your editor of choice..
(sudo) vi /etc/conky/conky.conf
Conky can be configured to display all sorts of weird and wonderful things, such as file storage, network use, CPU load, etc. - here's my own current display.
Most of the displays are self explanatory. Conky is one of those apps you can play around with until you get the effect you're after; the config settings are pretty clear. Click here for my settings file.
There's a stack of stuff around on how to customise conky and quite a following of enthusiasts. Go and have some fun with it!
Conky Home Page
Conky Configuration Settings
My Conky Configuration at Crunchbag Linux