Mac only: If you wish you had Windows' Aero Peek-like window previews in OS X's Dock, free app HyperDock brings them to OS X, along with other window management features.
HyperDock is a preference pane that adds a few handy features to your dock, most notably the ability to mouse over an icon and see all the open windows associated with that program. Clicking on any of those windows brings it into focus, or un-minimises it if it's minimised. iTunes and iCal have special pop-ups, iTunes' containing the currently playing song and controls, iCal's listing the day's upcoming events.
HyperDock also adds the ability to assign keyboard shortcuts to each individual app in your dock that let you launch Exposé, open Safari tabs, Finder windows and other such things. You can also resize windows, switch spaces and even emulate Windows' Aero Snap feature. Basically, it cures any Windows 7 envy that a Mac user might have.
HyperDock is a free download for Mac OS X 10.6 and up. It's currently in beta and it may cost a small, as-yet-to-be-determined amount of money after its final release.