Apple’s Mail.app only allows rich-text signatures, if you want an image in your signature, you could copy and paste it into the signature editor. But if you want links and other HTML features, you are out of luck. With the help of Spotlight, I quickly found the directory in which Mail.app stores the signatures, and they are in Web Archive format. So I opened up VIM in a terminal, hacked up some HTML, opened that file in Safari, do a Save As to Web Archive. Then I replaced the file in ~/Library/Mail/Signatures, it worked! 😀
Well, I guess I must warn that there are quite a few HTML emails haters out there, so don’t go crazy with the HTML signatures, or emails, or myspace profiles, or xanga pages. Oh man, don’t get me started on how lame most xanga pages are; but one quick advice: use CSS – don’t force text colors with HTML, if your hardcoded text in white in your posts, they look okay with “cute” dark backgrounds, but they become invisible in RSS readers…