You can send SMS from your yahoo or any other email. Every email provider has a domain name. Within that domain your phone number has unique identity.
If your phone number is 1234567890 and the domain used by the connection provider is abc.com then you can drop a mail to email@example.com. The recipient of the phone 1234567890 gets an SMS.
This service is available in India also.
Click here for international service providers.
Click on link1 or link2 or link3 for details on sending SMS Indian service providers (Airtel,Idea,BSNL and others)
I have already tried it so I can say this works for most of the providers.