If you're not taking advantage of using a good signature line, you're missing out on promoting your business every time you send mail. You should include your company name, URL, a business slogan, and a link to subscribe to your ezine or a freebie you may be offering. This is optional but helps add a flair of creativity to your signature.
Some people like to add a quote as in the example above.
It's best to make a few different signature lines and switch them every so often so you don't get bored. If your site runs special sales or promotions you can make a signature line just for that occasion. It works silently in the background selling your site and services. Signature lines are extremely important when posting to discussion boards and newsgroups. You can't just barge in and try to sell your products, but with a good signature line, you're doing it in an acceptable way when you post or reply to a message.