What is the best simple, lightweight web based scrum tool?

We use a simple google doc for our daily scrums. Keeps us on track!

If you want something simple and lightweight I suggest you to have a look at Agilo for Scrum http://www.agile42.com/cms/pages/agilo/ it is Open Source. It is based on trac, so if you already used trac or know out to manage it, you’ll have no problems. The Agilo interface is a great improvement over trac, and allows an easy administration and configuration.

HTH
tumbolz