I got this list from here. I just wanted to post this list here in case it goes away one day.
Free Email To SMS Gateways (Major US Carriers)
| Carrier | Email to SMS Gateway |
| Alltel | [10-digit phone number]@message.alltel.com Example: 1234567890@message.alltel.com |
| AT&T (formerly Cingular) | [10-digit phone number]@txt.att.net [10-digit phone number]@mms.att.net (MMS) [10-digit phone number]@cingularme.com Example: 1234567890@txt.att.net |
| Boost Mobile | [10-digit phone number]@myboostmobile.com Example: 1234567890@myboostmobile.com |
| Nextel (now Sprint Nextel) | [10-digit telephone number]@messaging.nextel.com Example: 1234567890@messaging.nextel.com |
| Sprint PCS (now Sprint Nextel) | [10-digit phone number]@messaging.sprintpcs.com [10-digit phone number]@pm.sprint.com (MMS) Example: 1234567890@messaging.sprintpcs.com |
| T-Mobile | [10-digit phone number]@tmomail.net Example: 1234567890@tmomail.net |
| US Cellular | [10-digit phone number]email.uscc.net (SMS) [10-digit phone number]@mms.uscc.net (MMS) Example: 1234567890@email.uscc.net |
| Verizon | [10-digit phone number]@vtext.com [10-digit phone number]@vzwpix.com (MMS) Example: 1234567890@vtext.com |
| Virgin Mobile USA | [10-digit phone number]@vmobl.com Example: 1234567890@vmobl.com |
Free Email To SMS Gateways (International + Smaller US)
These are all I could find from Wikipedia and other sources. If you’re aware of any other ones please share them in comments and I’ll add them to the list.
| Carrier | Email to SMS Gateway |
| 7-11 Speakout (USA GSM) | number@cingularme.com |
| Airtel (Karnataka, India) | number@airtelkk.com |
| Airtel Wireless (Montana, USA) | number@sms.airtelmontana.com |
| Alaska Communications Systems | number@msg.acsalaska.com |
| Aql | number@text.aql.com |
| AT&T Enterprise Paging | number@page.att.net |
| BigRedGiant Mobile Solutions | number@tachyonsms.co.uk |
| Bell Mobility & Solo Mobile (Canada) | number@txt.bell.ca |
| BPL Mobile (Mumbai, India) | number@bplmobile.com |
| Cellular One (Dobson) | number@mobile.celloneusa.com |
| Cingular (Postpaid) | number@cingularme.com |
| Centennial Wireless | number@cwemail.com |
| Cingular (GoPhone prepaid) | number@cingularme.com (SMS) |
| Claro (Brasil) | number@clarotorpedo.com.br |
| Claro (Nicaragua) | number@ideasclaro-ca.com |
| Comcel | number@comcel.com.co |
| Cricket | number@sms.mycricket.com (SMS) |
| CTI | number@sms.ctimovil.com.ar |
| Emtel (Mauritius) | number@emtelworld.net |
| Fido (Canada) | number@fido.ca |
| General Communications Inc. | number@msg.gci.net |
| Globalstar (satellite) | number@msg.globalstarusa.com |
| Helio | number@myhelio.com |
| Illinois Valley Cellular | number@ivctext.com |
| Iridium (satellite) | number@msg.iridium.com |
| i wireless | number.iws@iwspcs.net |
| Koodo Mobile (Canada) | number@msg.koodomobile.com |
| Meteor (Ireland) | number@sms.mymeteor.ie |
| Mero Mobile (Nepal) | 977number@sms.spicenepal.com |
| MetroPCS | number@mymetropcs.com |
| Movicom | number@movimensaje.com.ar |
| Mobitel (Sri Lanka) | number@sms.mobitel.lk |
| Movistar (Colombia) | number@movistar.com.co |
| MTN (South Africa) | number@sms.co.za |
| MTS (Canada) | number@text.mtsmobility.com |
| Nextel (United States) | number@messaging.nextel.com |
| Nextel (Argentina) | TwoWay.11number@nextel.net.ar |
| Orange Polska (Poland) | 9digit@orange.pl |
| Personal (Argentina) | number@alertas.personal.com.ar |
| Plus GSM (Poland) | +48number@text.plusgsm.pl |
| President’s Choice (Canada) | number@txt.bell.ca |
| Qwest | number@qwestmp.com |
| Rogers (Canada) | number@pcs.rogers.com |
| SL Interactive (Australia) | number@slinteractive.com.au |
| Sasktel (Canada) | number@sms.sasktel.com |
| Setar Mobile email (Aruba) | 297+number@mas.aw |
| Suncom | number@tms.suncom.com |
| T-Mobile (Austria) | number@sms.t-mobile.at |
| T-Mobile (UK) | number@t-mobile.uk.net |
| Telus Mobility (Canada) | number@msg.telus.com |
| Thumb Cellular | number@sms.thumbcellular.com |
| Tigo (Formerly Ola) | number@sms.tigo.com.co |
| Tracfone (prepaid) | number@mmst5.tracfone.com |
| Unicel | number@utext.com |
| Virgin Mobile (Canada) | number@vmobile.ca |
| Vodacom (South Africa) | number@voda.co.za |
| YCC | number@sms.ycc.ru |
| MobiPCS (Hawaii only) | number@mobipcs.net |