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