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Google Drawings is pretty good. I tend to use miro (formerly realtimeboard), since it has a couple of other features that are handy for map-games: a library of simple icons for a huge range of things, the ability to lock objects in place (and unlock later), and an in-built chat.

It has a couple of drawbacks, though - unlike Google Drawings you always have a white background rather than transparent, there's a text watermark in the lower-right corner, and if you're using the free version your export options aren't quite as good as Google Drawings. You can also only have up to a certain number of boards at a time, but that's less of a problem unless you want to keep old games around.