The only thing I hate more than HTML emails are HTML email signatures. I can forgive the use of simple HTML formatting inside emails, but adding images or doing fancy things with your email signature is just ridiculous (that includes plain text fancy things!).
One of my biggest arguments for not using HTML in emails is that I believe email is designed to convey information and that formatting should not be necessary to make it happen. Where else can you practically guarantee a block of pure information other than in an email signature?
Here is the signature I use when responding to emails for my web hosting company:
--
Raam Dev
Owner & Systems Administrator
Akmai.net Web Hosting
http://akmai.net
----------------------------------------------------
I have a couple of rules I try to follow when creating my signatures. First of all, signatures should only be 3 - 4 lines long and span a maximum of 72 characters wide. The signature should not contain your email address, since that's already found in the From field. You may include a phone number but a physical address should be reserved for your website.
The signature start indicator is a combination of three characters placed on a single line by itself. The three characters are two dashes and a space (-- ) and should be placed on a line by themselves. When an email client finds these three characters it changes the look of the signature so it appears separate from the content of the email. This makes your "information" more easily readable.
Remember, email clients have signature options built-in to help you avoid retyping a signature every time you send an email. When you design your signature, ask yourself if you would consider retyping the signature each time you send an email. If it would take you 3 minutes to retype it, then it's probably too big.