BuddyPress.org

phpfreechat looks easy enough to install and customize (css, avatars, etc).

The only major drawback I see is that it requires a page in order to work; meaning that users must remain on that page to chat.

In a network or multisite install, even a basic Buddypress site, a chat system needs to be independent of pages so users can visit other sections or network sites/blogs while they chat.