Mirrors are a great accessory to get if you are going to be riding in traffic and want to be fully aware of your surroundings! There are two types - handlebar end mirrors and clamp-on mirrors.
Bar-end mirrors fit into the end holes on your handlebars. Biktrix offers a set that is compatible with a limited number of bike models. They are also readily available from a number of 3rd party retailers.
Clap-on mirrors will attach to any free space on the handlebars with a clamping mechanism. These are generally long and adjustable so you can set them up in many different ways!