This one is nicely organized:

https://github.com/cubiclesoft/email_sms_mms_gateways/blob/m...

I have found this list helpful http://ram.kossboss.com/list-of-sms-mms-gateways-for-phone-c...

The other question is do any non-US providers have these gateways? Optus and Telstra in Australia don't support this kind of Email-to-SMS (at least, not for free).