In need of an OSX app for monitoring network usage

So there's a Mac on my network that has a tendancy to get used for a lot of Youtube.

Is there anything out there I can use which can keep track of daily network usage? Preferably with a toolbar icon of sorts.

I'm largely just needing something that the users of said Mac can look at to tell when they need to go and do something that's not eating up all the bandwidth on Youtube.