You can use the Email function from a tablet or PC to send someone a text as long as you know what carrier they are on.
U.S. Carrier SMS Gateway MMS Gateway
Altel @sms.alltelwireless.com @mms.alltelwireless.com
AT&T @txt.att.net @mms.att.net
Boost Mobile @sms.myboostmobile.com @myboostmobile.com
Sprint @messaging.sprintpcs.com @pm.sprint.com
T-Mobile @tmomail.net @tmomail.net
U.S. Cellular @email.uscc.net @mms.uscc.net
Verizon @vtext.com @vzwpix.com
Virgin Mobile @vmobl.com @vmpix.com
For instance: (222)-333-4444@txt.att.net
GRIM