I'll have to agree with Dvorak that mail apps that don't
allow HTML are lame lame lame. Apple's Mail will allow you to see HTML emails, but not compose them.
I guess that's supposed to be a security/annoyance feature, but sometimes I would really like to make an HTML email...
But at least we can have CSS for our signatures.
CSS, for those who don't know, is a way of styling web pages. It's more precise than plain ol' HTML, and quite frankly, more powerful and better looking. My personal site uses CSS exclusively (much to the chagrin of Netscape Navigator 4.7). Anyway, Melvin Rivera has a step-by-step tutorial on using CSS in your Mail.app signatures. You may want to learn a little CSS first, if you've never tried it before. Just like using a Mac, you may never go back.