Mac OS X only: Freeware application GrabUp automatically uploads screenshots to the internet and pastes the URL in your clipboard for quick and easy sharing. The application installs as a preference pane in your System Preferences and integrates with your Mac's default screenshot shortcuts. So if you hit Cmd-Shift-3 and select an area for a screenshot, it's automatically uploaded to the GrabUp servers and the URL is copied to your clipboard. You can enable or disable GrabUp through the preference pane, and if you enable GrabUp for your menu bar you can go through the history of your screenshot upload URLs. GrabUp is freeware, Mac OS X only. For more robust alternatives, check out previously mentioned Skitch and Jing.