I asked a similar question on SO.
While there is currently no official Windows 8 app directory, I came across the website Windows8Apps (though not perfect) that comes close. Update: As @kgander42 has mentioned, there is now a official search page for Windows 8 Apps.
Another alternative is to use Google or Bing to search for apps related to a keyword (for example, site:apps.microsoft.com foursquare) using the site:apps.microsoft.com operator.