Crisp is a web chat and messaging platform with uber power, the app has a knowledge base and help desk inbuilt and tons of integrations to make this an app to keep your eye on. Pricing is very reasonable and you can integrate it anywhere. We've used it in all of our chat implementations for software development.
Favourite feature: Magic Browse, help your website users out when they allow you to remote into their session from the chat app... no software required.
Other feature to note: If you leave your visitors hanging too long, they can play Crispy Bird, a flappy bird type game while they wait... it's addictive.