IM+ is also available on Windows Phones. It’s a Multi Messenger which supports a wide range of protocols. Even Skype. But the free Version shows Ads which can lead to a problem on smaller phone displays. But it’s a quite good Messenger.
Jabbim is a good alternative to IM+. But it’s not a multimessenger and it doesn’t run in the background. If you close Jabbim, its closed. Good if you have an account just for joining some chatrooms.