CloudApp can do this for screenshots or any file in the finder, plus copies a link to the clipboard: http://www.getcloudapp.com/ (not affiliated, just a very happy user)
Can't all screenshot tools do this, just by you telling it to save them into ~/Dropbox ?
I haven't found another one that automatically puts a URL to the file at Dropbox on my clipboard.
What chrisbolt said, but this also notifies you via growls, allows you to control whether each screenshot is posted and allows you to specify a filename. :-)
And, of course, copies the link to the clipboard.