Climsy is an easy-to-use desktop tool that allows you to share your clipboard images at a click.
The program works by running in the background, and monitoring your clipboard. When it detects that you've pressed PrtSc, Alt+PrtSc, or just pasted an image there, it pops up a window with a "Share" button. Click this and Climsy will automatically upload your image to Climsy.com, ImageShack or Dropbox, depending on how it's configured, copying the image link to your clipboard when it's done.
And if you don't want to share the image, there's no need to manually close the Climsy alert: it'll automatically disappear in a few seconds, which means the program never gets in your way.
Climsy works well for the most part, and is extremely light on resources, using only 440KB of RAM (private working set) on our test PC.
It doesn't support many upload destinations, though, so if that's a problem then you may want to consider something like JShot, which allows you to capture, annotate and edit your grab, then upload it ImageShack, Dropbox, Picasa, Twitpic and more.