Windows: Windows 7 shipped with a much-needed snipping tool for taking partial screenshots, but it's not exactly loaded with features. Snipping Tool++ adds a few handy features that connect your screenshots to the cloud.
Once the app is installed and residing in your system tray, hitting Ctrl+Shift+3 will allow you to drag a box over a portion of your desktop and take a partial screenshot to save locally. If you feel like sharing your screenshot, taking it with Ctrl+Shift+1 will immediately upload it to Imgur and copy the URL to your clipboard. A handy optional setting even allows you to immediately annotate the image before saving or uploading it.
If you take a lot of screenshots, the importance of a tool like Snipping Tool++ cannot be overstated. It's available for free from the developer's website.