Network Speed Monitor is a simple app that lets you monitor network activity. Once installed, the application will run on the menubar. It will display both upload and download speeds. This information will only be available when you are connected to a network. If you click on the menubar icon, you will find a menu that lets you change some settings. The only two things that you can modify are the refresh interval and the unit type. By default, the information is refreshed every 1 second and the unit used is bytes per second. You can change the interval to anything between 1 second and 10 seconds, and if you want, you can change the unit to bits per second.
I tested the application by doing things like downloading torrents, watching videos on YouTube and moving files on my network. The application provided accurate readings most of the times, but in some cases, when I was transferring data from the Internet, the readings were very close to 0. This happened when I was watching a video on YouTube.com. Most of the time the readings were near 1000 kilobytes per second, but mid-way through the video, the meter went back to 10k, and I could see the video was still buffering, and quite fast, by the way.