I am currently working on a Google App Engine based CRM app, which now has a preview of this feature : Bulk-email, using email-templates..

http://blogs.ifreetools.com/2010/05/feature-preview-email-te...

The app is not open-source - but free with resource restrictions and ads. Custom deployments are available for as low as $99/year (yes, per year), which enables you to use the free quota (upto 2000 emails / day) and purchase additional resources as required directly from Google.

Works with Google / Google Apps accounts.