DockView is a desktop customization app that lets you take a neat trick from Windows and mimic it in Snow Leopard: with DockView running, when you mouse over an app's icon in the Dock, you can instantly see live thumbnail images for all the windows open in that app.
You can accomplish something similar without DockView, by clicking and holding a Dock icon for a few seconds, but DockView gives you a much quicker and less intrusive view, while allowing you to quickly flip between different apps in the Dock. Plus, DockView's ability extends to the Mac OS Application Switcher, the horizontal ribbon of apps you see when you hold down Command-Tab. When a thumbnail pops up, you just click on it to pull up that window, and keyboard junkies will be glad to know that DockView also shows a quick key combination you can use to pull up a given window. (For example, if you have three windows open in Mail, DockView will display three thumbnails with a key combo you can use to pull up each: Command-1, Command-2, or Command-3.) In addition to DockView's central schtick, it provides some nice extras like mini playback controls in iTunes (and similar apps) and, in the Dock, a list of the day's appointments in iCal.
You can tinker with DockView's preferences for more options (for example, to only show thumbnail previews when you're holding down a hot key, such as Control or Option), but many users won't need to go beyond the defaults. DockView is easy to try out with a free trial, although you'll need to register if you want to remove a fairly persistent nag screen.