HyperDock (€6.95 – about £5, hyperdock. bahoom.com) adds new features to the Dock, perhaps the most useful being previews of each window that’s open in an app when you place the pointer over its icon. It also enables additional functionality for specific apps when you hover over their icon. For example, Calendar displays upcoming events and iTunes displays album art and playback controls. Holding down å while clicking an icon switches to Mission Control for that app's windows (similar to how Exposé worked in older versions of OS X).
Finally, HyperDock features handy shortcuts for automatically resizing windows. Holding ≈+å and tapping the arrow keys resizes the active window and moves it to the left- or right-hand side of the desktop. It adds Windows Snap-like shortcuts to OS X as well.