The service is called UserTesting.com and you can get a test for as little as $19. I have already tried it a couple of times and found small changes that have already made a big difference on the site I was working on. I got two 15 minute videos the same afternoon that really showed some issues with how well the site did at conversion (and how people simply don’t read on the web!)
If you are familiar with user testing, you will know that subsequent small groups of testers are better than one big group all testing at once. Test with 3-4 people, fix the problems, then re-test. This is because a small group will find 80% of issues, but they are often "show-stoppers". Once you deal with them, you need to re-test.
I really recommend checking out usertesting.com. Maybe joomla.org should spend a few thousand there testing Joomla (an excellent use of donation dollars!)