I've previously used LivePerson. Not difficult to put into the site and pretty easy to use.
If someone's on it during the workday, it'll chime when someone is requesting a chat. If you are offline there are options to forward questions to an email address or alert your mobile phone.
They can also customize their chat window to fit with your site design and layout, etc...
Not sure if it's the best but pretty reasonably priced and easy to use.
