Android: Disa is a messaging app that aims to pull all of your IMs, texts, picture messages and video messages into one good-looking interface that's fast and easily accessible. It's in beta right now and has some drawbacks, but it's looking good so far.
I've been using Disa for a few days. The interface is clean, the app is fast, and SMS and MMS blend beautifully with WhatsApp messages. Similarly, there's no shortage of settings and options to keep your messages organised the way you want them. You can toggle emoji and switch services quickly without starting a new message. You can read more about the app's features in this FAQ.
In a way, Disa is what we'd like Hangouts to grow into. One interface for the messages I send friends, the photos I snap and fire off to them, but also one place to see the SMS and MMS messages that go through my carrier and are sent right to my phone number. Right now Disa supports SMS/MMS and WhatsApp, one of your favourite alternative texting apps, but the developers are working on adding more messaging services as soon as possible, specifically Hangouts and Facebook Messenger.
To give the app a try, you'll need to first join the Disa.fm Google+ Community, so you can stay up to date on improvements, report bugs and offer your feedback on the app's development. Then, you can join the testing group here, while you're logged in to the Google account you use to install Android apps. Finally, once you're in the testing group, you'll be able to download the app from Google Play here. You must join the testing group at least before you can download the app though.