tawk.to - FREE, the best free one around!
Also check out livechatinc.com, they have a plan for 'startup' companies, where they give you the 'Team' edition for FREE for 1 year to help you get started. After 1 year, you revert to their 'Pay Per Chat' version, which is $1/chat.
If you have a lot of chats, that can get expensive, so tawk.to might be the best option to help save you money.
If you are looking for paid, LiveChatInc.com and PHPLive are by far the best live chat solutions I have found over the years and then tawk.to if you are looking for FREE and feature rich.
Good luck with your live chat search! Hope this helps.