The available telecommunication networks are Dialog GSM,Mobitel,Etisalat,Aritel,Suntel,Lankabell,Hutch and Sri Lanka Telicom.
Here Suntel wOw is a good free SMS provider website.
The Suntel wOw site www.wow.lk you have to register first to send free sms.
After complete registration you can log into sms.wow.lk page.
Then you will see a web mobile phone with number like 074XXXXXX.
Where you can send free sms.But you have to get a "YES" reply message from sms receiver.
After adding the contact Suntel automatically send a sms that you are trying to contact please reply "YES".
If you got 'yes' you can send it for free.
Advanteges:This truely send sms to sri lanka.Fast delivery.
Disadvantages:Long run,Registration,Confirm Registration,Adding contacts,Getting yes reply,Send free sms.

Here are list of sites which you can send free sms to sri lanka.