I've tried WhatsApp, GroupMe, Kik, Google Voice, LINE, Trillian, and basically every other messaging app you can think of — I've even tried using Skype as an IM service, because at least my parents know what Skype is. I'd given up on finding an app that had all three things I need: cross-platform, useful online and off, and accessible enough that I could convince everyone I know to use it. But Google's new Hangouts app is all that and more.
There are Hangouts apps for iOS and Android, plus Gmail and Chrome. You can message with one person or a hundred, either via text and emoji or on a video call, and if you're offline your messages wait for you to get back on. Hangouts is already baked into your Gmail — there's nothing new to sign up for, just a new app to download. It shows you who's online and who's not, and whether or not they've read your last message. You can send pictures back and forth, and they're saved forever in a Google+ album that's only accessible to the people in your Hangout. It's not perfect — we're still waiting for voice calling, SMS integration, and even more platform support – but it's the fastest, simplest, most integrated messaging app we've used yet. And its best feature is its sales pitch: "Mom, it's like texting but you can do it from your Gmail." Done and done.