OpenStreetMap is a collaborative project to create a free editable map of the world. The maps are created using data from portable GPS devices, aerial photography, other free sources or simply from local knowledge. Both rendered images and the vector dataset are available for download under a Creative Commons licence.

OpenStreetMap coverage often includes features not found on other online maps, such as municipal boundaries, individual buildings, recreational trails, and abandoned railroad and interurban rights-of-way,
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Walls looking a bit sparse? Have access to a plotter? Check out the Big Map Blog, which features a new downloadable high-resolution historic map every weekday. It's like Shorpy, but for cartography geeks.
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New Yorkers often say their city is the center of the universe. ExtendNY, brings this belief a bit closer to reality. It's a online app that extends the iconic Manhattan street grid to every point of the Earth's surface.

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Colorbrewer is an online tool designed to help with the selection of color schemes for maps and other graphics. It's quite useful if you need to find distinct but complementary shades of the same color for zoning and land use maps, GIS projects, and the like. ...