You can do it through Yahoo messenger, as long as their cell phone carriers are on the list.
You all have to be signed up for Yahoo messenger too. You just have to download it to your computers, and then you just log into it and click on their names to send/receive texts.
It is call SMS, but it is actually sending out texts, I do this all the time with my kids. I do not like to text on my phone LOL :D:D:D