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. :)

Remote testing http://usertesting.comhttp://verifyapp.comhttp://skype.com

For recruiting users http://ethn.iohttp://wufoo.comhttp://intercom.io

And Evernote for taking notes and sharing with team.

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.

http://silverbackapp.com/http://inspectlet.com GA

https://usabilla.com/http://silverbackapp.com/