Graph My Run – run useful and analysis (locally) on a (local) gpx file

WebPlotDigitizer – upload an image of a graph in order to estimate the underlying data

Unicode Shapecatcher – draw a character in order to identify the best-match unicodes for it (detexify does the same for LaTeX)

RGB Color Codes Chart – look up a color in RGB

pdfcalendar – customize a simple calendar, and download it in pdf

IMSLP – public-domain sheet music