WhatsApp is a cross-platform messaging app that lets you send unlimited messages, photos and audio notes to your friends and family over a 3G or Wi-Fi connection. The best part? Messages you send won't count against your carrier's text limit. Even better, service on Windows Phone is free for the first 12 months (though you'll have to pony up $1 for each subsequent year thereafter). Other robust features include group chat, GPS location data, and automatic switching between SMS and WhatsApp.
Free for first year, $0.99/year thereafter