How to check "Phone storage" usage?

If you are getting the "Phone storage low" warning from your Android device, the easiest way to find out how much phone storage is left and what is taking up all that space is to use DiskUsage.

When you first run it, it will ask if you want to view "App Storage", or "Storage Card". Select "App Storage", and it will scan and display a simple but effective visualization of your phone storage.

The above is a visualization of my Nexus One phone. It shows that I have 196.2MB internal storage, of which 13.6MB is taken up by system data, 140MB is taken up by applications, and I have 42.7MB free. When the free space reaches 10+MB, you will start getting "Phone storage low" warnings.

A click on the second or third columns lets you zoom into further so that you can explore the storage utilization of the various applications:

This shows you the applications that are taking up the most storage. So they should be the first ones to target when trying to increase the amount of free phone storage.