google.com/hangouts
Hangouts is Google's cross-platform text and video chat service with apps for iOS and Android, integration with the Gmail and Google+ sites, and a Chrome extension that runs on Windows, Mac, and Linux PC's. If you have a Google Voice number, you can send and receive calls using the Chrome extension or the iOS app, with Android coming soon. SMS and MMS support, like for iMessage, has also been leaked.
Notifications sync between all installations, and there's hundreds of hand-drawn emoji (smileys.) You can also see where each person in the conversation has read up to, and when they're typing. You can group video chat with up to 8 people on any platform, and even save that video to YouTube.
Essentially, it's the best chat service ever and is going to get a massive, massive upgrade very soon. BBM is dust.