I just launched a small app I built over the holiday at
http://blast.designfound.com
It is a simple multipart email designer that will compile your
plaintext, HTML markup and CSS stylesheet code into a functional
multipart email. You can then send this email to a few recipients to
test how the design performs. This is not a total email newletter
management service right now. I just needed it as a tool to help some
clients design the multi-part emails (they send them out themselves with
their email applications).
Feel free to use it, and hammer on it for me. Thanks to the community
for making this easy to develop over my holiday.