Net Notify

Net Notify will give the current data transfer rate as a notification. It gives you the speed of both Wi-Fi connection and Mobile Internet.

Also, you can see the Total Data Transfer log. Start the log, Stop and Reset it whenever you want.

It's one of the most light weight app which doesn't need to connect to any server to know your Internet speed. Just start the monitoring any time to see speed at any point of time and stop whenever you want.