Screen Courier is an easy-to-use program (from the developers of System Explorer) to help you take screengrabs, edit them, and share the results online.
The program must be installed; there's no portable option. But there's no adware, either, and the setup process is quick and easy.
Once running, Screen Courier takes over the PrtSc key, and works in exactly the same way: pressing PrtSc captures the full screen, while Alt+PrtSc grabs the current window.
Whatever you're capturing, a small notification pane appears afterwards with two options.
Click "Edit" and your image appears in Screen Courier's built-in editor. This is extremely basic - you can highlight an area in your grab, add text or draw a freehand line, and that's about it - but, if you just want to spend 20 seconds roughly annotating something, it'll do the job.
Click "Share" when you're done and the image is by default uploaded to Screen Courier's own site, and assigned a link, which is copied to the clipboard. Paste this into an email, forum post or instant message and anyone else will be able to take a look.
There's no registration required, no personal details, and no obvious limit on the number of images you can take and share. The Screen Courier site also says that grabs are automatically deleted if they're unused for a "certain time period", though, so don't rely on the service archiving them for long. They could disappear at any time.