Open Heartbeat
on SourceForge. Visit the SourceForge project page

Open Heartbeat is a new open source project aiming to provide first-class tools for real-time data distribution - especially tailored to stock quotes as part of an automated securities trading system

Open Heartbeat daemons can be seen processing several thousand messages per second while taking only a few percent of a machine's CPU.

Open Heartbeat is available on Linux and Windows platforms, as either console applications or running as a background daemon or NT Service

Open Heartbeat is scalable, lightweight, simple to deploy and administer and fast. - Oh - and free.