Windows: If you're looking for a lightweight—193k!—and portable screen capture tool without tons of features you'll never use, ScreenCatch is a good match. Tiny, portable and simple to use, ScreenCatch not only captures your screen but shares it too.
Download ScreenCatch, run it—no install necessary—and a little Earth icon will appear in your system tray. Click on the icon and it will capture your entire screen. In the editing screen that follows you can crop your screen capture before saving or uploading the files—uploaded files are shuttled to the ScreenCatch servers.
One caveat for multi-monitor users: ScreenCatch will only capture the primary monitor. While you can drag the editing window to another monitor and use the capture button to keep snapping captures of the primary workspace you can't grab a capture of a secondary or tertiary screen. That's definitely an oversight we'd love to see corrected in a future version of the application given how light and snappy it is otherwise.
If you're looking for a screen capture tool with more features, check out our Hive Five on the topic Five Best Screen Capture Tools. ScreepCatch is freeware, Windows only.