Try YouWriteOn. Rather than me tell you how it works, this is from their About Us page:
YouWriteOn began in 2006 to assist new writers to develop their writing. Editors for leading publishers such as Random House and Orion - who publish bestselling writers such as John Grisham, Bill Bryson and Ian Rankin - review our highest rated writers each month.
YouWriteOn's premise is simple: members upload opening chapters or short stories and the YouWriteOn system randomly assigns these to another member to review. You then review another member's story excerpt - assigned to you at random - each time you want to receive a new review back in return. After 5 reviews a story enters our chart system and the highest rated writers receive free feedback each month from editors for leading publishers Orion and Random House.
Feedback can be diverse, but the experience of the site over its history has been that collective feedback from a range of reviewers helps writers to get a collective viewpoint of what works well and what needs developing in their stories. The result has been that the majority of stories are higher rated as a result of revising their writing through feedback.