I've been tracking the "conversational UI" trend on my blog... post here with background and examples, and a few more examples. These aren't all based on messaging, but a lot are.

One of my favourites is Digit, which helps you save money entirely through messaging.

Edit: Thanks for the kind comments! I'm always on the lookout for more examples so watching this thread like a hawk and please do send more my way :) Contact deets on the blog. (I'm currently prototyping a simple Telegram bot for a client, there are so few design patterns, it's fascinating)