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.
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.