It is a bit hard to decide which to use since there are so many chat tools provided and they are all free. I could at least say that the number reaches more than 60 chat tools ready to employ and embed to blogs or websites.
Oops! I forgot to present one other tool; that is: Shoutmix. The service enables you to modify the appearance of your widget by employing many optional themes, as well as custom CSS. This tool is able to filter profane words from being displayed on the shout-box.