Google retired “Send to Phone”
I used to use Google Send to Phone service and the FireFox plugin to send SMS. But today I found that free SMS service has been stopped by Google. I remember that Yahoo had a similar service before. It turns out Yahoo Web2SMS is not longer available either. I do not know when that happened.
Here are a list alternatives I know:
(1) Via Email
AT&T: number@txt.att.net
T-Mobile: number@tmomail.net
Sprint: number@messaging.sprintpcs.com
Verizon: number@vtext.com
AllTel: number@message.alltel.com
Where number = your 10 digit phone number.
I did not use this method for quite a while, I hope those emails still work.
(2) Via carrier websites
They often provide simple web forms to send SMS. For example Sprint has messaging.sprintpcs.com, T-Mobile has t-mobile.com/MESSAGING.
(3) Gizmo SMS
GizmoSMS.com site allows you send SMS globally. You can pick the country name from a long list.

This is really a shame — I used this service all the time to send little notes to my phone, or to quickly send a friend an SMS. I wonder if abuse was the problem?
Yes. You may try GizmoSMS. So far it works great for me. It sends SMS with some ad.
I am dissappointed because this is a super-useful feature! Bring it back!