Mac: Screenshot apps aren't hard to come by, but Share Bucket offers a lot of flexibility.
Say you're perfectly happy using the default Cmd+Shift+3 and Cmd+Shift+4 on your Mac to take Screenshots. You can configure Sharebucket to automatically upload any new captures to the cloud storage service of your choice, and copy a shortened URL to your clipboard in the background. If you want the app to take a more active role, just replace Cmd with Option in the standard shortcuts, and Sharebucket will open your screenshot in an editor with a full suite of annotation tools.
Share Bucket is conceptually a lot like Skitch, but makes it easier to choose how you want to interact with your screenshots. It doesn't try to force you to use the app in a certain way, and gives you lots of options to set it up to match your preferred workflow.