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

Features:
✓ Simple and elegant menu bar based UI.
✓ A gauge in the menubar to indicate your usage level.
✓ Monitor total usage in a rolling 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.
✓ Location-based monitoring.
✓ Selectively watch only internet traffic, excluding your local usage (e.g. Time Machine and file transfers to other Macs on the same network).
✓ Selectively monitor specific network interfaces.