Vocareum's platform allows teachers to create assignments which are graded by peers. Turn the "Peer Grading" option ON in the "Options" sections of the Assignment.
You will need to tell the system how many peers you want each student to grade and the rule for aggregating all the grades into the final grade. Currently, average, median and max are the supported policies for aggregating peer scores into a final score. As with any grading, peer grades can be overridden by someone with Teacher/TA privileges. Once all the assignments have been submitted, the system randomly assigns the peer(s) to each student. The student to be graded shows up as a button in the grading section for the student. Once the student finishes grading that student, he/she is assigned the next peer until the student has graded the required numbers of peers. One-at-a-time assignment grading minimizes the impact of a student not participating in the peer grading.