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http://fivesecondtest.com/

http://www.usertesting.com/

http://usabilla.com/

http://www.testgang.com/

http://feedbackroulette.com/

http://www.loop11.com/

http://feedbackarmy.com/

http://www.usaura.com/

http://www.conceptfeedback.com/

http://www.pickfu.com/

http://ethn.io/

I experimented with http://rifff.com/ a bit for crowdsourcing feedback. I got some great detailed feedback, but I think the site is more prone to X vs. Y questions, which are helpful as well.

I found http://criticue.com on HN a few weeks ago and I've received some very good feedback from it.

Feedback Army was my best competitor: http://feedbackarmy.com/