https://scrumy.com/about - OSE's lightweight scrum platform.
OSE is using the most light-weight, wiki-embeddable, cloud-editable, no-permissions, free scrum platform. It does not appear to be open source, but it's the lightest weight package that we know of. It tracks tasks in different threads - through the 4 stages of backlog, in progress, to be verified, and done.
One way to use Scrumy as a Burndown Tool would be to post all the tasks of the Development Spreadsheet in the backlog, and as they are placed in the Done column - that corresponds to a burndown - which can be graphed on a weekly basis.
- Are you logging?
- Are you putting in links to work product?
- Do you have an assignment?
This is another explanation of scrum, from another software provider:
- Simple, lightweight, free, easy to use - especially after watching above video for the theory.
- Embeddable - see Flashy XM for an example
- Cloud editable
- Not active development, last blog post - July 2010 - http://blog.scrumy.com/
- Private boards have to pay
- List of about 50 open source scrum tools - 
- Burndown tools ?