This may be an old one, but it’s a hack that’s had a chance to prove its usefulness. If you use AIM or iChat, it’s extremely easy to send a message to someone’s cell phone via SMS. Just add a the complete cell phone number preceded by “+1” to your buddy list.
For example, if your friend’s phone is 612.555.1234, the AIM ID you should enter is +16125551234. If the message is to a phone outside of the U.S., just include the country code.
Any messages that you send to that AIM account will be converted to an SMS message. The recipient will also be able to reply via SMS and it will hit your IM account. Your recipient’s carrier may charge for SMS service, so make sure it’s okay to be sending messages before you start ringing up somone’s phone bill.