Aggregators are good not just for things like news, movie, and game ratings, but also for local knowledge. Yelp, Gowalla, and Foursquare have all made a business out of this, but up until now you've had to spend time bipping back and forth among them to glean knowledge from each service's members.
That's changed now with Fourwhere, a service that launched , which now combines reviews and check-ins from all three services in one place. The end result is a single page you can visit to see the most recent local user activity at any given time.
Fourwhere's entire interface is controlled through a Google Map, which can be scrolled or zoomed to whatever neighborhood, or part of the city you want. It employs two different ways to find out more information about venues: either letting you look at comments about it, or just things like its address and phone number--the latter being pretty useless since the first option shows you these things anyway.
In my brief use of the site this morning, I found its performance to be sluggish. Results were slow to load, and even on larger displays with a big area for the map, it only loaded up small clusters of listings, which needed to be manually refreshed if the search radius meandered into a nearby area. I also found it was, in several cases, missing results from both Gowalla and Yelp when there were tips and reviews there--something I wouldn't have known if I hadn't ventured off the site to go find that information on my own.
Despite the current shortcomings, I really like the idea of having one place that aggregates not only the tips from these sites, but, more importantly, the check-ins. When done right, and given a sense of time, Fourwhere could prove itself as a very powerful tool for showing what's hot and what's not based on a much larger group of users than any of the three services could offer on their own.