Here are four sites, using which you can send SMS messages that are up to 500 characters long in India.
Chikka Messenger is a service from Indiatimes.com. Using this service you can send 320 characters long SMS to a list of 200 friends. You can also add your Gtalk buddies and receive Google talk messages.
Chikka allows you to send messages from both desktop and mobile. It has a desktop application for Windows and a Java app for Linux and Mac users.
Visit: Chikka Messenger iBibo iSMS
iBibo lets you send 340 characters long SMS in India. To send messages you have to create a group and add your friends. You can message everyone in your group by texting a message from your mobile to the site.
SMS7 lets you send SMS upto 440 characters long. The site is simple, just register your mobile and a password will be sent to you via SMS. Login using that password and your mobile number as ID.
A good thing thing about this service is that, you can schedule your messages and it will be automatically delivered at the set time. You can also set personal reminders with it. SMS7 has a limit of 50 SMS per day.
BollywoodMotion lets you send 500 characters long SMSs in India. No registration is required. Just enter the recipients number, message content and your name. Then hit Send SMS.
Know any more sites? Mention it in the comments.