Several free on-line services meant for sending SMS to mobile phones are in place.
The web based SMS service offered by Indyarocks is one among them. To make use of this service you need to register and verify your mobile phone number.
Another such service for sending SMS from your PC or web is Chikka messenger. You have to register with your mobile phone number for this service also.
If you do not have a mobile phone number and still want to send SMS then you can try Atrochatro.
If you also want to send voice messages in addition to text messages and accommodate landline numbers too then you can try the newly launched voice broadcasting service, Msgforyou.The advantage of this service is that you can send voice messages to a group of people at a specified time in one go.