Windows: There are a lot of great screenshot tools for Windows, but SnipDock offers something special. It will clip any part of your screen and auto-dock it on the side so you can quickly refer to it later.
You might come across a paragraph you have to transcribe, or maybe you just want to choose between two images. Whatever be the use for needing to refer to one part of your screen without Alt-Tabbing, SnipDock is the easiest solution. Hit the "Snip Screen" button, drag the area you want to clip and you're done.
The dock is always-on-top and maximised windows will use it as the margin. You can choose whether to dock it on the left or right, or to leave it floating. The image can be zoomed in or out with the scroll wheel, and the dock adjusts its width accordingly. Snipped images are auto-saved to your Pictures folder.