Note: This question is intended to complement an earlier question on installed, desktop software.
What GIS-related, or even non-GIS-specific, web- or cloud-based software/tools/services -- that are free -- do you find useful, whether for GIS development or application?
For example, I'm finding that Google Docs (aka, Google Drive) increasingly useful for general productivity and Google Maps is always useful as an street atlas or as a globe.
A specialized example is Q-Cogo, useful for coordinate geometry.