TrafficBot is the only network usage monitoring app you will ever need.

Features:

• Simple and elegant UI.
• A gauge in the menubar to indicate your usage level.
• Monitor total usage in a rolling (very nifty feature for college students!) or a fixed period.
• Notify you when usage reaches your limit.
• Run scripts or open any documents or apps.
• Log and graph your usage.
• Graph is detachable, so you can always see your current usage.