Windows Phone iconography is a key part of the Metro look so the SDK provides a number of icons that match the look and feel of the built in apps on the phone. If you have the tools installed they are located at C:\Program Files (x86)\Microsoft SDKs\Windows Phone\v7.0\Icons

But what other icons are out there floating around? I went through the MSDN Samples and the Training Kit and found a few more that may prove useful (reviewing/interpreting the EULAs for the samples is left as an exercise for the reader)

Here is a list of where I found it and what it was used for in the sample. The images on the right are NOT the proper transparent PNGs. They are screenshots of the images because I had some issue with the blog rendering the transparent images. In any event you should go check them out at the source anyway.