Here is something I use quite often, texting from your computer. the only thing you have to know is who their mobile carrier is. once you know that you simply send them a email from your computer and they get it as a text. if they reply it goes back to your email address that you first sent it to.
find their carrier below and simply add thier phone number to the email address:
AT&T Wireless (now part of Cingular)
[10-digit phone number]@mmode.com
[10-digit phone number]@mobile.mycingular.com OR
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