I learned today that AOL Instant Messenger can send text messages to US phones.
Just send an IM to the phone number, including area code, with a +1 in front of it. So for example: +1(123)456-7890.
When the recipient replies, that shows up right in the IM window.
Neat! Might save me a bit of money, given that Verizon charges 10 cents per message sent or received.