Windows 7 has a cool feature where you hover your mouse over an icon in the Task Bar and a preview of the windows belonging to that application pop-up in a preview; allowing you to quickly switch to a specific window if you have more than one open in that app. It’s one of the few features found in Windows 7 that I wish was built-in to Mac OS X. Fortunately, there’s a System Preference utility available that brings that feature to OS X.
HyperDock (free while still in beta) gives you that capability and more. Because HyperDock is a Preference pane, no icon for it will clutter your Dock, and uses relatively little system resources to do its job.
HyperDock offers application window previews in Mac OS X's Dock
HyperDock allows for plenty of preview bubble customizations when hovering over Dock icons, including size of previews, a close button, and more. When you hover over one of the Dock previews for a second or so, the window itself appears on screen at full size as well.
Among some of the other cool features of HyperDock is what I consider a killer feature, Window Snapping. With the feature turned on, moving any resizable window to the left, right or bottom edge of the screen automatically resizes the window to fill half the screen and docks it to that side of the screen. Moving a window to the top edge of the screen resizes the window to fill the active area of the screen – leaving room for the Dock to remain in view.
There’s plenty to like in HyperDock, and hopefully it will be priced reasonably when it ventures out of beta.