Crisp provides a simple solution to communicate with current and potential customers that visit your website.
Zendesk provides solutions from live chat to knowledge bases, it can help with your customer service like no other.
Groove provides solutions from ticket management to a knowledge base and more, and what makes it even better is it's great for small businesses.
Freshdesk is your friend which provides services from ticketing to a multi-channel helpdesk.
Tawk.to is a completely free live chat service that is easy to install and easy to use.