Frozen Pea Friday is a demonstration of how Twitter can be used by nonprofits. People on Twitter change their avatar each Friday to be something about peas ("peavatars") to draw attention to the Frozen Pea Fund, a fundraiser for breast cancer research inspired by the cancer experience of Susan Reynolds. The Frozen Pea Fund clicks through to the American Cancer Society Making Strides Against Breast Cancer donation site.

For more about how this came about check:

Frozen Pea Fiday, Twitter, and the American Cancer Society

and

Frozen Peas -- Why It Works