10000s of nunit tests => best tool to distribute them across vms to run them faster?

[Tom Whittaker]

For my .Net application (C#), I've been using NUnit. The problem I'm
now trying to solve is that it takes too long to run all of the 10000s
of tests on a single machine. I can distribute them using categories
across vms.

What I'm looking for is a great proven tool that can take my existing
nunit tests and then distribute the load across a vm farm based on how
busy each vm is.

Does anyone have any strong recommendations for fantastic tools to
use? (commercial or open source)