Skitch has been my go-to image annotation tool for the last couple of years. Unfortunately, Evernote has announced that in January they will no longer support the Windows, iOS, and Android versions of Skitch. The Mac version will still be supported. If you’re looking for a replacement for Skitch, these are the tools that I’m going to use now.
Alternatives to Skitch for Windows
On a Windows computer you can use the Snipping tool to capture all or part of your screen. From there you can use the built-in tools for drawing on your screenshot.
For a long time I used Jing to take screenshots on my Mac and on my Windows laptop. I’ll probably return to using that on my Windows computers. Jing allows me to select a portion of my screen, highlight image elements, draw arrows on my screenshots, and type on my screenshots. The only reason I had stopped using Jing was that I preferred the drawing tools in Skitch.