When an instructor uses class time for student work, rather than lectures, very often the active learning is structured around activities that can be done in small groups (from 2-5 persons). There are a number of good reasons to do small group work as opposed to individual work. (Another possibility is to structure individual work in the form of competitions, where using clickers can be very useful.)
The members of small groups have to be motivated, and one technique for ensuring motivation is to use low-stakes peer evaluation. This can become a logistics nightmare for the instructor. Imagine in a semester 15 small group activities, in a class of 40 students. Each class will generate 40 students writing peer evaluations of three other students, if they are in groups of four. This is 120 scores to enter, in a complex way (the scores are organized by pair rather than just one name). Also, the groups have to be assigned, class time is taken up with students re-grouping into their groups, and contingencies (what if people are absent and a student is left solo?) have to be dealt with. I believe strongly in random assignments each session for the group work. Students then get to meet most of the other students in the class, and are not stuck in a bad group for a whole semester, which can be very demoralizing.
Fortunately, there are some online free tools available to make things run smoothly. They demand a little bit of investment, but will pay off nicely. Students generally love small group activities, and they understand how low-stakes peer evaluation keeps people focused (on the work and not on their smartphones).
One of the best online tools for managing peer evaluation appears to be TEAMMATES, a resource developed at the National University of Singapore, and distributed online as an open-source tool available to everyone. Here is there introductory video:
Another similar peer evaluation tool is PEAR from the University of Guelph. Here is their introductory video:
There are many peer assessment tools that measure more complex aspects of group participation. Here is a nice one with a business school orientation. Here is another document with numerous good examples of peer assessment tools.
A number of sources post excellent resources for thinking about best practices in peer evaluation processes. A resource from the Foundation Coalition gives a clear overview with many pertinent examples.