Eventative is a cross between Groupon and Evite; it's a website that allows you to generate invitations for tentative events. It addresses one of the most serious pain points in event planning as it doesn't obligate either the host or guests to participate unless a minimum number of guests commit by a specified time, thus relieving pressure on both parties. It allows for a significant increase in events (parties, trips, rallies, sports games, reunions, etc.) to be seriously proposed without risking a planner's social capital.
*The site is no longer operational.