Constructive criticism is beneficial to both giver and recipient, but the fact that people often can't deliver negative (and sometimes positive!) feedback without social risk discourages the behavior.
Admonymous allows people to anonymously admonish and admire their friends and acquaintances. All you need to do in order to let others leave feedback for you is sign up and publish the link where people can see it, like in your email signature or in your Twitter bio. By becoming aware of your blind spots, you become more able to address them.