The folks at HowAboutWe do a decent amount of HTML email, as you might expect for any site where people can send each other messages. Dissatisified with the tools for developing HTML email, I wrote EasyMailPreview, a Ruby gem that makes it very easy to see the results of an HTML email in development mode. It auto-discovers mail methods and method arguments, and lets you write them on the fly with in an admin-ish screen.
I've been using it for a while, and I find it eases the pain of developing HTML emails. Somewhat.