HyperDock is a System Preferences pane (currently in free beta) that lets you add some time-saving functionality to your Dock that is similar to the Aero Peek feature found in recent versions of Windows.
With HyperDock installed, you can mouse over applications in the Dock and see thumbnail previews of all windows open in that app--which can make switching between different apps and windows faster and easier. You can accomplish a similar effect without using HyperDock (by just clicking and holding an app's Dock icon), but HyperDock shows you the preview much more quickly, and in thumbnail form, just above the Dock. HyperDock lets you close, select, and even resize windows from this minimal view, and you can fine-tune the appearance of the thumbnails in a variety of ways. HyperDock also provides some useful, app-specific features and keyboard shortcuts (along with tools with which to create your own), such as playback controls for iTunes and a stacked list of your iCal appointments for the day.
HyperDock is an excellent value during the free beta, and we look forward to seeing the final release (which "will cost a small fee").