Listingbook has been a favorite of mine. They used to provide the service free, through the Los Angeles and Marin County MLS's. They have now begun charging something around $150 per year. I like the service, since I can see which one of my client's are actively searching for homes, along with the dates and times. Not all MLS providers have allowed Listingbook to be implemented, so check with yours to see if it is available. Hope this helps!