Send and receive text SMS messages from the PC for free.

Google has a option in gmail where you can put a phone number into your chat box and send a text message to any phone in the US.

This is how you get it set up:

In GMAIL click:
Settings,
then Labs,
then scroll down to SMS chat which is fairly far down the list.
Select enable
Then back to the top to save settings.

Now you can simply type a phone number in the chat contact box. It will offer to save the number with a name associated.

Then simply type a message and wait for a reply.

These messages are not free to the person receiving the message.