I just started using Twitter, I am planning to use it to market the iPhone applications I am developing. Twitter can be a great marketing tool if used as such, it has the potential to reach a lot of people and it is very simple to use. Many people might argue that Twitter is not for businesses but I disagree, it is word of mouth at its best.
Try it for yourself, create an account in case you haven't created one already and make sure you have the name of your company and website in your profile. It is a good idea to start writing small bits describing your products or services, and as you start following people in Twitter, start asking them to promote your service or product, you'll be surprised with people's willingness to help you and your business. You can also use it to write news about your company, product releases, offers, etc...