DrQueue works on several platforms (Linux, FreeBSD, Irix, Mac OS X and Windows) and with many rendering engines (the following are supported in the GUI: Maya, Mental Ray, Blender, XSI, Lightwave, Mantra, Turtle, BMRT, Shake, After Effects, Aqsis, Nuke, Terragen, 3Delight and Pixie [other engines are also supported but need some more scripting] ).
It is distributed under the GPL license, Open Source.
Offering commercial support contracts, custom development and professional services.
DrQueue is fully scriptable. Either through the render shell scripts, through it's python bindings, or a combination of both. (Now also including ruby bindings as beta release)
DrQueue includes DrKeewee, a small server that allows you to check the status of the queue through a web browser. DrKeewee has been completely written in python using the python bindings.
There is a master that acts as central server, where all tasks are stored and where nodes running slaves report their status.
The slave runs on every node of the renderfarm. They execute some checks to decide if their available for rendering on not. If available they request a task to master. They report their status periodically to the master so it check timeouts and reassign tasks.
drqman is the GUI used to control jobs and computers. You can reprioritize jobs, stop them, restart specific frames, change frames to be rendered and so.
DrQueue has been tested in production environments and has proved to be a stable alternative to other privative products, with the advantage of being licensed under GPL and commercially supported.
Check the site www.drqueue.org for more information.