Max Ogden asked about Google Maps-like things for astronomy images, free alternatives to Google Sky. I foolishly said there were none. Happily the author of Chromoscope corrected me. does a nice job with different spectra. The source images are kind of low res though. It’s all on GitHub though and the image sets are downloadable, so it wouldn’t be hard to roll a different viewer.
I wonder what the state of the art in non-Mercator slippy maps are? Raster reprojection was a mess last I checked.
He also pointed me to http://www.wikisky.org/ aka sky-map.org