Windows ' Mac
Put the PrtScn (Print Screen) button to actual use. LightShot is a tiny utility that takes over that button. Push it and you can pick what part of the screen to turn into a screenshot, plus annotate with tools like text and arrows. It's also available for leading Web browsers to capture what's on a page. Once captured, you get choices to save to a file, the clipboard, the cloud, social networks, or to just search for similar shots in Google.
Windows ' Mac ' iOS
Skitch is a pretty good screen-cap tool that truly excels at annotating those images with big, fancy arrows to point out what's important.
Windows ' Mac
As much service as it is software, Jing is all about creating a place to share images (and short vids) you take with the utility. It's part of the Screencast.com site, where you can share your captures, which itself allows up to 2GB of stored screens for free.
Another of the Icecream apps that rock, Screen Recorder freely handles capturing everything on screen (even a still image)—but it will also capture one small section moving on your screen, like a playing video or game. The drawing panel lets you add text or arrows to the screenshot or video.
You need Java running in your browser, but if you do, it doesn't get easier. Visit the site, click "Launch screen recorder now, " and you'll be instantly capturing your screen and voice. Instantly share it on social media.