If you use OpenTable

When people cancel reservations at the last minute, HotTables texts would be diners that the coveted table has opened up. Launched by OpenTable, it functions like a real-time wait-list that's basically a more official RezHound (a free desktop or mobile service that scans OpenTable for 11th hour reservations). The app is still in its pilot program, so interested users can request an invitation here. There is a charge of $1 per seated diner and you must be a VIP OpenTable user, which requires making at least 12 reservations each year.