We are an NGO that would like to use SMS to send out reminders for events and feedback tool. Which providers are supported by CiviCRM?
Out of the box, you have two possible providers, clickatell and twilio.
Both seems to work fine, you should check directly on their website because the cost depends on your location, the location of your recipients and the volume you want to send.
Besides Twilio and Clickatell, folks have written extensions for other gateways - but of course they may be out of date. I know of these offhand: