Unlimited data plans are being killed off by mobile carriers, one by one. Even home Internet connections aren't truly unlimited. Monitoring apps, whether for your smartphone or for your computer, can help you find out how much bandwidth you're actually using.
NetSpeedMonitor (32-bit | 64-bit) is a small Windows program that runs in the taskbar and displays your data usage at a glance. It's particularly useful for tracking your data usage if you tether your PC to your smartphone. If you're a Verizon customer, you may want to check out CNET's FAQ on Verizon's tethering settlement with the FCC.
Clicking on NetSpeedMonitor from the taskbar will display your upload and download data for the month, day, and session. To display your historical data by day or month, right-click on NetSpeedMonitor from the taskbar, and select Data Traffic.
You can also display active port connections with NetSpeedMonitor, configure the layout, and change the bitrate measurement unit.
That's it. Keep in mind that NetSpeedMonitor monitors your network adapter, but can't distinguish between LAN traffic and Internet traffic. If you use a home media server and stream data to your PC, NetSpeedMonitor will count that traffic in its usage totals.