160by2.com is a reputed site that we have used in past to send free SMS's to any mobile in India. No ad's are inserted into the messages, It's fast and free. Registration is required, but it is fast and hassle free, once you get the activation SMS on your registered mobile, you can login and click on the "Send SMS" link

This opens a form where you can enter the mobile number to which you want to send your message and the message itself can be typed into the appropriate field.

Click on Send Now/Later and your message will be delivered to the recipient.