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For the Baltic Sea area, downloadable shapefiles of protected areas, pollution, shipping, fisheries, and some background data, from the Helsinki Commission. (Site is currently under maintenance but click here for the same data).
From the Ordnance Survey, the national mapping agency. Includes OS VectorMap™ District (mid-scale buildings, roads, railways), OS Street View® (simplified street level map), and Boundary-Line™ (electoral and administrative boundaries), among others.