Photon is a free tool for capturing, annotating, processing and sharing screenshots. (And it can capture desktop videos, too.)
Capture types are, well, limited. There's no full screen, no "active window" capture, all you can do it press PrtSc and draw a freehand rectangle.
Once you've done that, the area remains highlighted and a couple of toolbars appear around it.
You can annotate the image with simple drawing tools: a pen, line, arrow, rectangle, highlighter, text caption.
The results can be shared on Twitter, Facebook, Pinterest and VK, uploaded to the developer's free online storage service for instant reuse (you'll get a link you can post anywhere), or saved locally.
Click the Google icon and a browser window opens to maybe identify this image (name a person, say), and display similar images.
Clicking "OCR" extracts and displays any plain text from your image.
There's also a tool for recording a video of your desktop activities. This can optionally show your mouse cursor and a mouse hotspot, and may be saved as MP4, AVI or OGG.
Photon's core screen capture tools are basic, but strong sharing support and some useful extras (OCR, Google's "find similar images") could make it worth a try.