A free, open source map of the sky, adaptable (see form below) and zoomable/draggable with mouse/gestures. Done with the help of the d3.js visualization library and the GeoJSON data format. Try it out below, but make sure to maximize your browser window and click/tap 'Full width' to enjoy it in its full glory!
Hint for projections: Try orthographic or stereographic for hemispheric projections, equirectangular, Hammer, Mollweide or Winkel-Tripel for all-sky projections, Bonne, Colignon or Foucaut for something fancy.
Download the sources from GitHub, or clone them if yo have git installed, or grab the archive 'celestial.tar.gz', if you just want to play locally with the app. That may require a local web server to work, depending on what browser you're using. Firefox should be fine, Chrome only loads local files if you start it with a ' --allow-file-access-from-files' command line parameter, Internet Explorer AFAIK doesn't do that at all. For a local web server try XAMPP, or if you have node.js installed (which you need anyway to compile d3-celestial), simply install the http-server module.
Since it uses HTML5 canvas, a fairly modern web browser is required, IE above version 9, Chrome/Firefox/Safari/Opera are all fine in any recent version. If the zooming and panning is a bit slow, try reducing the magnitude limits. On mobile browsers the zoom-gestures clash with the regular UI functionality, I've yet to figure out how to avoid that. Meanwhile, tapping the zoom buttons on the upper left should work just fine.