This is a free windows network renderer for Maya that supports the Maya renderer and Mental Ray

$150 per machine
supports Maya, Shake, MR, Prman, LW
can be scripted to support any renderer that has a commandline
runs on Windows, Linux, Irix and OSX

Maya®: Secrets of the Pros™
By John Kundert-Gibbs
Aprox US $40.00
ISBN: 0-7821-4055-6
Here's a windows only client/server distributed renderer for Maya that comes with this book and allows you to render on as many machines as you like. Comes with source code so it could easily be made to do otherthings as well. MUSTER.
I think the demo version let's you render to your hearts content on two computers and then is $620 for unlimited computers on NT and UNIX(IRIX & LINUX?) or $770 with a year of tech support.
Best value for mixed platform renderfarms with 3+ computers.
Slow to download DEMO
!!Muster now supports MacOS X!! LEMONPRO
I'm not sure what the DEMO allows but I recall that it was $150 per computer.
Best feature it's crossplatform. SMEDGE 2
Windows Only. Render Pools. Error Correction. UI & Command Line.
$100 per computer. Good value if have 4 or less computers.
Site license for $5000 (good if value if you have 50+ computers).
Best feature is that it will support other net renderable applications.
Two week Demo RENDERMAX
They've got GOD on their side.
$595 for first 5 license, $150 for additonal packs of 5.
Best feature is the Image Scatter which allows the network to render chunks of a single image.
Good value if you only have 3+ Windows machines.
Downloadable DEMO SPIDER
Windows Only,Pools/Group,Error correction
Supports Maya, Lightwave or Chalice
Pro version Best features: Supports Image Slicing and command line scripting
This is now free for unlimited usage.