When Should I Visit Tate Modern takes Foursquare check-in data and shows what days of the week are less crowded at various museums, galleries, and theatres in London.

Also trying to leverage Foursquare checkin data for end user utility, but at the opposite end of the spectrum from the aforementioned website: Where the Ladies At, an iPhone app that tracks high concentrations of the fairer sex in San Francisco.

I'm glad some companies are trying to extract some consumer utility from all that checkin data, because I get very little personal value from Foursquare, Gowalla, Yelp, and other checkin services.