The feature I most thought dumb before Leopard came out but turned out to be one of my favorites is QuickLook, which lets one preview the contents of a file from the Finder without actually opening the file. Never had I realized how badly I really wanted to browse a directory of cryptically named PDF files and figure out what was actually in each one, or find out just what I had typed into the half dozen files all cryptically named "notes.txt".
Leopard comes natively with the ability to view a number of file types, leaving many types unrecognized and displayed as generic icons in QuickLook. Fortunately there's a plugin-api and many developers creating plugins. They give you the ability to peer into zip files, eps files, Adobe Illustrator files, and even Commodore 64 image files (thank goodness!). One particularly great idea is "Suspicious Package," which allows you view the contents of installer packages before running the installer (it's not that hard to do with the Xcode Tools install of PackageMaker, but this is nicer).
Since the api appears to provide a lot of flexibility in how the contents can be represented, people are creating nicely functional interfaces within the display view, such as indented directory views, formatted and colorized code, and buttons and sliders and all that.
QuickLook Plugins has many good ones. With that name, they'd have to be the definitive source, but I'm sure there's more out there.