Google Voice works extremely well for SMS - including via email. If you configure it to forward an incoming message to your email address (not just a notification, the actual message), you can reply directly with the email and the recipient will receive it as an SMS. There is also a lightweight email-based interface where you can access your address book, etc. and send SMS from there. If you have Internet access this is by far the best option.

I've had reasonably good success getting it to work for non-US SMS destinations also - although this can be a bit hit-and-miss and occasionally messages disappear into a black hole.