Talk.to is a messaging app which supports Google Talk, Facebook Chat and also their own PingPong service.
The service currently supports Android, iPhone, Mac, Windows and PCs via browser, and the team has announced support for Blackberry and Windows Phone will be coming soon.
The app allows one to share files, create groups and has delivery confirmation. Future plans include support for voice and video, and sending SMS to groups.
Read more about the announcement at WindowsPhoneApps.es