£2.99 PeakHourApp, peakhourapp.com
PeakHour and iStat Menus both provide graphs to display inbound and outbound network traffic. However, PeakHour can track your usage if you’re on a fixedbandwidth internet account. It pays to use the free PeakHour Compatibility Check app before buying PeakHour. However, if your kit’s compatible and you can leave a tracking Mac running, PeakHour could help curb excess usage.