NextShout was a commercial shoutbox product I released two or three years ago, for the vBulletin forum software. Its bandwidth consumption is far lower than any competing products, due to its use of efficient JSON and only sending information deltas, rather than completely refreshing entire blocks of HTML. Other features include a modular command system, multiple channels, automatic pruning and logging (configurable per channel), and new post/thread notices. It requires PHP 5.1.4, and at least vBulletin 3.6.9 (currently unavailable for 4.x).
Find more information and the download at its website.