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If you've been struggling to find good third party applications for Last.FM, worry no more, as they're now gathered in one place: build.last.fm.
There are only four categories, which doesn't seem enough to me: For your Desktop, For the Web, For Social Networks and Scrobblers. True, it's not a problem now due to the relatively small number of available apps, but if this number ever grows to several hundred or more, better categorization will be needed.