Notifu will allow you to send a message to an individual or group, confirm the message was received, and get responses to act upon.
Today, Notifu is 100% free (including sending SMS and voice messages to anywhere in the world); however, it is likely in the near future that they will have a Free Edition and Pro Edition and some features will require the Pro Edition (like sending SMS messages, sending voice messages, etc).
The following browsers are supported:
Internet Explorer 6.0
Both Registered Users and Unregistered Users have access to the same features; however, an Unregistered User cannot access their Notifu messages and contacts after leaving the http://www.notifu.com site.
The purpose of the Try It Unregistered button on the home page is to provide a quick and easy way to try Notifu without even registering.
Their Wiki serves as a FAQ resource.