Vlingo, a Cambridge startup, doesn't market itself as a product for drivers, but I'm including it on this list because it certainly can assist you while driving, and it's representative of a growing genre of similar smart-phone applications.
With Vlingo Plus, you simply push and hold down a button while issuing your command - "Text Helen: Let's meet for lunch" or "Twitter: I'm dying for chocolate" or "Web search: dominoes." You then press send and the app carries out your request. For sure, Vlingo has its limitations. You can only contact people on your contact list, you can't listen to incoming texts and e-mails, and outgoing texting and e-mail isn't available for all phones yet. But as an easy, send-only service with a one-time fee of $18, it has potential.