I use Zopim chat and have done for a couple of years. It's actually really good chat, what i like about it the feature where it hides when you're not logged-in. There's nothing worse than chat being installed but your customer's see no one online so this feature is great to avoid that.
EDIT: Looking at tawk.to it looks good, does anyone know if it supports SSL pages (https://)?
If so I'm going to test it out