If you haven't seen it yet, one of the cool new things about Mac Mail in Leopard (at least I think so) are the new "stationary" templates you can use for writing email; basically they are HTML emails. Unfortunately 90% of the templates are unusable by professionals; they're mostly fun and cute looking. However, with a little work, you can convert some of the templates and rework them to serve your needs, i.e. looking and being professional.
I found a cool link at a site called TechnoSanity in which Jeff Turner explains how to rework some of the Apple stationary. With a little time in Photoshop and minor HTML knowhow, you can make something you'd be proud to attach your name to and clients will like to see. To find out more, visit the TechnoSanity article by clicking HERE.