I often get my students to edit each other's written work. I find that his assists both parties involved as they get to see another example, acquire some new ideas and hopefully see either the correct or incorrect way of doing things. This feedback is often very powerful. Generally, I limit this to essay assignments but I can now see that this can be extended to other types of assessment as well.
The simplest way I have students edit each other's work is simply to do a "continue/consider"activity. In other words, I will have each peer review to make note of 2 things that the writer should continue to do (positive reinforcement) and offer two things the author might want to consider doing or changing (constructive criticism).
I would be very interested in learning more about a web-based peer/self-assessment tool such as Web PA. One of the criticisms of group work is that all group members receive the same mark. Using Web PA, the group members can influence that mark by reviewing the efforts of group members and honestly reporting to the educator so that they can adjust the mark for each individual student.