You can already do this from carrier websites, and from your own email, but you have to know what service provider your friend uses, and what the proper syntax is to send a message from your PC. Teleflip has solved that problem. I just tried this service out. It's so simple, it's beautiful... and it's free!
Open your email client. (Yahoo, Gmail, Hotmail, Outlook, etc.)
Enter recipient address: eg. 4125556789@Teleflip.com
And that's it!
Tips & Warnings
Your friend can respond to your message from their phone, and it will be delivered to your email account!
The only down side to the whole service is that you can't send a picture as an attachment, but it's still cool.
Currently, Teleflip works only in North America (Canada, US, Caribbean, Hawaii, Guam). They are working on adding more countries soon.