Dan , you can also use your headset and fly on http://www.vatsim.net
with real ATC and other pilots over voice. Pretty advanced and you have to know what you're doing but there is training available and alot of reading material.
It is also pretty hard to set up...if you have any queries feel free to e-mail me.
Other than Vatsim , as Tim has said Voice Buddy can be used. Although , voice recognition is still in early stages and not as good as its thought out to be...yet.