This is US carriers, sorry guys, but basically this is an email address you can send an email to and it gets converted to an SMS for your recipient. Saves SMS send charges, at least.
Alltel: [Phone Number]@message.alltel.com
AT&T: [Phone Number]@txt.att.net
Nextel: [Phone Number]@messaging.nextel.com
Sprint: [Phone Number]@messaging.sprintpcs.com
T-Mobile: [Phone Number]@tmomail.net
Verizon: [Phone Number]@vtext.com
Virgin Mobile: [Phone Number]@vmobl.com
Feel free to add ones for Europe... I'm sure they exist.
Pulled from a pcmag article
http://www.pcmag.com/article2/0,2817,2339983,00.asp