This is where Apple has an opportunity to really fix some weaknesses with iOS as well, and OS X could be the place to experiment with it.
Share sheets are just like they are on iOS. Apple even uses a similar button.
Click the button and you get options for sharing whatever it is you are looking at. In Safari? Why not tweet that page to your Twitter account? Useful, and something I use in iOS almost every day.
However, Apple could take this to the next level by allowing 3rd party developers the opportunity to add their own applications to the share sheets. Are they going to? I have no idea, but I hope that Apple takes a cue from Windows Phone 7 (what a TERRIBLE name) and adds the ability for programs to proclaim what they are useful for (I think they are called “contracts” in WP7).
There is room for improvement, of sure.