Cropper is good Windows software. It's a simple, modern screenshot utility that makes it easy to save images to disk or upload them to various services. I've got mine feeding Flickr. Mac users have Skitch, now we don't need to be envious.
The key thing is it's simple. Run it in the background and it overrides PrntScrn, Alt-PrntScrn, etc to automatically save screenshots to a folder, upload them somewhere, email them, whatever you'd like. If you click the tray icon you get a simple UI for selecting a rectangular region of the screen instead. Windows' default UI is copy to clipboard, paste into some paint program, crop, save, launch an upload program, upload. This is one step.
In addition to Cropper itself you want the plugin pack. The SendToFlickr plugin works great and I imagine the Twitter, Facebook, etc integration is good too. (Don't install TFSWorkItem; it crashes Cropper). The only feature I'm missing is playing a sound on screenshot.