Email signatures are great. They get attached to all your outgoing emails and can be a simple, passive, means of advertising. There are definitely more effective ways to market, but since creating a signature takes all of 5 minutes at the most it's worth the time.

If you use an email program like Outlook or Firebird you already have easy tools to create an attractive signature but if you use web based mail you may not be able to create an html based signature. However there is a solution for web based email services if you use Firefox or Chrome. A little while ago I found Wisestamp which is an extension for Firefox or Chrome and is compatible with Gmail, Yahoo Mail, AOL, Hotmail, and several more services. It has a nice interface that makes creating a signature very easy with some nice tools like social media buttons and the ability to display items from an RSS feed.