One of the most valuable tool for virtualization is free and comes from Microsoft.
It's called Microsoft Assessment and Planning (MAP), it's a capacity planning product that supports hardware and application virtualization platforms, and it's able to agentlessly discover and take into account over 50,000 computers.

The version 3.0 released in April supports both Virtual Server 2005 R2 and Application Virtualization (formerly SoftGrid) 4.5.

The new version 3.1, now in private beta, is 64bit and supports Hyper-V.

Enroll for the beta program here.