Official speedtest.net app to test your internet connection.
This is available in the Windows Phone store. http://www.windowsphone.com/en-us/store/app/speedtest-net/4fcd4de1-050b-44dc-b123-a786808eb49b
jon kent commented
finally came out!
Kevin Hyatt commented
good to show some numbers and compare (like "mines faster"), but this is it.
Bandwidth is a decent app, but I prefer speedtest.net
You can use bandwidth for that (although speedtest.net will be better since it's available on all devices and makes it easy to compare results)