Now a new site called callwave is giving ability to send five sms every day to any mobile in the world. You need to register before your can send sms.
First when you register they ask for your mobile send you a sms which has the a code which you should put it on the site to confirm. Once this is done you can start sending sms five of them for free. Even international sms.
If you are using vista they have very cool sidebar gadget which you can use. And its just one click away for sending sms.
I am using this service and have tried sending it to my friends in India and works flawlessly. i have yet to try internationally.