Today, people ask their friends "what're you doing tonight?" via SMS, twitter, IM, email and more. There is no single, dedicated application for making and broadcasting plans. So it's impossible to see what people are doing tonight in aggregate.
Use doingtonight to make plans for tonight. Posting plans on doingtonight is dead simple, and browsing plans is addictive. People are most social between 5pm and 5am, so we focus on casual and uncalendared plans for tonight only.