Handy and well-designed site: Speedtest
Speedtest.net is a broadband speed analysis tool that allows anyone to test their Internet connection. Ookla provides this service for free to anyone curious about the performance of their connection to and from hundreds of locations around the world. Whether you test just for fun or you really need to certify and validate the true speed of your Internet connectivity, Speedtest.net is the place to be. You can view all of your historical results, share them easily, and even compare them to others in your immediate area or around the globe.
Update: there is a simple explanation of the meaning of internet speed here.