Experience sampling is a method of collecting process-report data -- that is, data that represent the ongoing mental processes of the individual -- as they occur. A principle advantage of process-report data is that, rather than relying on the individual's recollections, the data provide a picture of the person's mental process that is less influenced by memory. That is because feeling states, cognitive states, and other ongoing processes are recorded at the time they occur.
Through the impressive contributions of the experience sampling community, an open-source software program is now available to researchers. To find out more about the experience sampling community, and more about their program ESP, please follow this link to their site.