NetSpeedometer is a bandwidth measuring app for the Mac. The application is a big speedmeter that tells you the bandwidth usage of your Mac's network adapters. In my case, I connect to the Internet via Wi-Fi, so my WLAN adapter is the one being measured. NetSpeedometer shows two hands, one for upload and one for download. The former one is red and the latter one is blue.
The speedmeter shows the bandwidth usage in real time, and it also keeps a log to tell you how much you have downloaded and uploaded since the app was launched. The hands of this speedmeter move a bit slowly for my taste, so you might not see increases in bandwidth usage. For example, I did a speed test on speedtest.net, and the hands moved to the correct speed with a 2-second lag. NetSpeedometer never really showed the maximum speed I achieved on that test.
The window cannot be resized and there aren't any configurable settings. You can run the .app file from the DMG image and it will work just fine.
Although the website tries to sell you this app, you can use it for free, and there is a disclaimer that appears when you first launch it claiming that it is indeed a free app and it can be freely distributed.