I would love to hear, what tools (digital/analog) you guys use for usability testing.
We used Silverback and it proved to be quite useful. It put us back in the concept mode for 2 months after the first usability test.
Note that it's as important to get usability right. You need to conduct 3 tests at different points in time with a set of at least 5 people. It's better to have no more, no less. This will cover 85% of the issues you might have with the project you are testing.
The sooner you do the first test, the better. We had to change as much as arrows in a thumbnail that users confounded with navigation. :)
http://usertesting.com seems to be the biggest player in the space. They charge $50 per user (for a 15 minute test).
I've been using them with success and they seem to have the most exhaustive pool of candidates to draw from.