Snappy acts like an OS X Add-On,
accessible via ⇧⌘2 (Shift-Cmd-2).
If triggered you'll notice a crosshair which you can use to select the area you what to snap.
Your snap is now pinned above all windows, always visible and always accessible.
Oh, and you can access your snap history by using the "Snaps From the Past" option in the menu.
You can share your snap very easily, just right click the snap an select how you want to share it. Currently there are 2 options, via Snappy and via Facebook.
Snappy-app.com will provide you with an URL that you can send to anyone.
After logging in to Facebook from Snappy, you can share you snap by clicking the "Share on Facebook" menu item in the snap's contextual menu.
That's it! Start snapping!
Available on the Mac App Store