Back in November, Mail.ru Group, the new owners of ICQ, announced that they would be developing a client app for Windows Phone. In addition to its namesake, ICQ interfaces with many popular chat platforms, including Facebook chat, GChat and AIM. Users can conveniently sign in to all of them at once using just their mobile number and receive push notifications. ICQ also allows for automatic adding of contacts right from the Address Book and retains a history of conversations.
ICQ can be downloaded for free here.