Broadcast Messages is a feature of WhatsApp that lets you send a message to many contacts at once, without them knowing who the other recipients are. The app also supports Broadcast Lists, which let you preserve a saved list of contacts to whom you regularly broadcast messages.
WhatsApp itself is a mobile messaging service that lets you text without paying for SMS, and send multimedia messages via your device's wireless connection. It has a free year-long trial period, after which you have the option of extending your subscription for $0.99 USD per year. Its maker warns that the good ol' trick of reinstalling the app and free-riding for another year won't work.
Apart from Windows Phone, WhatsApp is available for iOS, BlackBerry, and Android as well.
Download WhatsApp: Windows Phone