PowWow is a free simple service for scheduling user research using your Google Calendar. The fine folks at Pointless Corp (aka Viget) created this as a side project for handling scheduling of user interviews and testing, but could be used for any kind of event.

There are other apps like this that remove the fuss of coordinating schedules via email. PowWow does it simply, and provides a polished and pleasing experience. You pick the times that work best for you and send out a calendar with available appointments and any additional information to potential participants. Participants can select the appointment time that works best for them and you will be sent a confirmation with their contact information.