You should take a look at
CloudShare Pro. It's currently in free beta.
We've been testing it out for our own investigations into virtualization and have found that it works great for our own testing and demo purposes. You are able to create an "environment" with up to 6 VMs and then provide temporary access to that environment to others.
You're able to create snapshots of the environments so that you can allow users to do whatever they want without fear of damaging this initial setup.
Very useful on a number of fronts: testing various combinations of Adagio versions, environment/OS troubleshooting, benchmarking, webinar play space, etc.