Free listing is a technique for gathering data about a specific domain or topic by asking people to list all the items they can think of that relate to the topic. It can be used to gather data in large group settings or in one-on-one interviews.
Free listing is used by cultural anthropologist to understand particular aspects (domains) of a culture and subgroups within the culture, but it is quite useful for understanding users, tasks, terminology, and other issues important to HCI practitioners. You might use a method like free listing at a customer conference to ask your audience to "list all the things about our product that frustrate you." You can then plot the frequency and position of responses and use that as input to your requirements and design activities.
Free listing is highly cost effective. It provides a large amount of data in a few minutes and does not required trained facilitators or special materials, software, or hardware.