Key features of the app include the ability to chat with contacts from multiple IM networks (Mail.Ru, GTalk, Facebook, Odnoklassniki, StudiVZ, and AIM), support for push notifications, automatic adding of contacts based on your address book, full chat history maintained between app sessions, and the classic ICQ sound set.
Windows Phone Store description:
Connect to your friends and family, no matter where they are.
What's new in v1.8?
— Windows Phone 8 support.
— Sign in easy by a phone number, and be in touch with your friends and family.
— Photo filters.
There are effectively two versions of the ICQ app available for Windows Phone: ICQ and Agent + ICQ. The first of these is the "classic" version of the app. The second integrates Mail.Ru Agent instant messaging service into the app, while maintaining the classic ICQ experience.
Mail.Ru's Agent service adds a number of extra including the ability to send free SMS messages using the app (Russia and CIS only) and the option to post messages to a microblog. However, it is only widely used in Russia, so those living in the rest of the world would be well advised to download the plain ICQ version of the app.
Both apps are available for both Windows Phone 7 and Windows Phone 8.