Your signature is especially important in business emails. Make sure your name and/or company name is listed so the recipient will know who you are.
It also helps to list your phone and/or website in case the recipient needs to call you or refer to something on your website.
Most email programs will even let you include a signature link or image in your emails. Just be careful with images, they don't always come through (especially if the recipient has their mail client set to "Text Only") and you don't want to include a huge image file that eats up your recipients bandwith.