To SMS someone's cellphone from a computer:

1. Figure out what carrier they have.
   • FoneFinder.net. However, NOTE that number portability means that this data is sometimes wrong.
   • carrier-specific pages
   • SMS them ALL, carefully record which emails bounce, and based on which one doesn't bounce, conclude that that's that person's carrier (download this helper spreadsheet)
2. Find the website or email address for that carrier's SMS gateway, and use it to SMS them
   • Wikipedia
   • hiptools.net
3. Alternatively, try a "universal" SMS gateway
   • [free] Teleflip's FlipOut, shut down on September 2008
   • [free] SMSeverywhere.com, only T-mobile/Sprint/Nextel
   • [pay] Skype can do it