I used to use teleflip to SMS from my computer. It was Sweet! I could send out emails from my email client and teleflip would take care of the whole "what carrier?" piece.
The "what carrier" is the deal-killer question when it comes to sending texts to cell phones from the computer. I know my friend's phone number, sure, but I have NO IDEA who he gets service from. Come on!
So there are a couple of services online that do let you send SMS texts without specifying which carrier, and as I was testing them out I found that this month Google introduced a little add-on to Gmail that is likewise very sweet. You have to enable it in the Settings--Labs section, but once it's enabled you can chat from your Gmail screen with your friends' cell phones.
Most importantly, you don't need to know their carrier. Google handles that part and gives you a convenient little chat window so that your friend (student, coworker, etc.) replies and you see it as if she were on another computer and engaging in a little chat. Very nice.