Notifu is a free service that lets you send messages to either an individual or a group, via text message, IM, email or voicemail (or all) and then track when/if the message was read and gather responses. You can compose a message through the Notifu website, then decide how to deliver it to your contacts. If it's voice, Notifu's text-to-speech service will deliver a voicemail. Then, you can see who received the message. You can also offer up response options and see in real time who is responding and their preference.
Aside from being a nice way to contact a group of friends, or schedule a business meeting, this could be a good way to send a quick survey, or poll your website visitors about a new product or service, or to notify users of new announcements, etc. You can try Notifu without registering at their website, fresh into public beta.