Almost every good service is mentioned here except mine. ;)
In December, 2012 I launched a new service aimed at website improvement based on feedback from fellow founders and web designers. It's a one-for-one exchange. You get reviews in exchange for reviewing other sites.
Reviews are short (~50 words). A reviewer is shown a snapshot of the page he or she is supposed to comment upon but also has an option to open it in a new browser tab for a more detailed examination.
Based on the feedback I received from the users, the majority of reviews are surprisingly useful, especially considering they're anonymous at the moment. It seems that the authors of websites submitted for critique are looking for quality feedback and are willing to do their part to help others in return.
Curiously enough, the look of the service itself is far from perfect; I launched it as an MVP less than 2 months ago. The new stunning design is in the pipeline. :)
Please do have a look and send me your feedback. You can reach me on twitter @martinbilski