If you’re looking for a Mac app that takes screenshots, lets you annotate them, uploads them to the cloud directly, and does it all for free, Skitch is probably the first app that'll come to your mind, and what most people would recommend. Skitch has more than just the few features listed above; in fact, as far as screenshot apps for Mac go, it is possibly the most feature-rich free app of its category that we've come across. Though you might really like it for all its features that could save you a lot of time, or you might really hate it for all the features you'll probably never use and the controls that aren't very easy to figure out. For a simpler solution albeit one with a price tag of $2.99,
Glui is a great alternative. It’s an app available in the Mac App Store that lets you take screenshots (of a particular area, the whole screen, or just one window), annotate them, draw on them with a pen tool, add some basic arrows or rectangular shapes to them, and upload them to your Dropbox account, without having to use separate apps for the purpose, or use a heavyweight app like Skitch that offers many features you don't require.
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