With a domain like map.pr, you'd think this would be another link shortener. Instead, it uses the Foursquare API to give you a great way to visualize popular places on FourSquare. And, if you're following the World Cup, you can also use Mappr to find spots frequented by fellow fans.
Mappr lets you filter locations by categories, such as shops, nightlife and sights. I imagine it would be a great way to get a local's view of a new city. You can also discover the spots in your own city that are popular. Mappr's icons, each representing a venue's category, change size depending on its relative popularity. It appears to take into account the neighborhood's aggregated popularity, so that you aren't overwhelmed by large icons from the hip hoods.
During the World Cup you can also filter by teams. This lets you see where your fellow fans are checking in, so you can join them. Or, look for rival fans so you know to stay away.
There's also an iPhone app, but based on my tests I'd recommend skipping it. The web version is useful enough, though it's a shame there isn't a good way to get the same info on-the-go.