Leopard has now only been available for a week, so even if you waited in line to purchase the latest Mac OS, you're probably still getting used to all the new features. My personal favorite at this point is probably Spaces, because I can split up my work into different areas and quickly switch between them. The addition of Stacks is a welcome change, too, because I now have all my articles in the same place--right there on the Dock. One surprise for me, though it probably shouldn't have been, was that I soon realized I missed a certain program immediately when I went to save a document.
One of my old favorite Mac programs doesn't really change anything major about the Mac interface, but once I got used to using it, I realized I couldn't live without it. Default Folder X adds a slick yet unobtrusive button interface to the right side of your open and save dialog boxes. You can use the buttons to quickly save a file in common locations like your most used folder or standard locations like the desktop or your user folder. You can also create several personalized folders which show up in your list so it's easy to get to your favorites without the need to dig through the Finder. As an extra bonus, you can add Spotlight comments so you'll be able to quickly find files you have tagged in a certain way. I should point out that Default Folder X works just fine on Tiger, as well, so don't fret if you haven't taken the leap to Leopard.
Maybe its a small thing when compared with the great new stuff in Leopard, but I was glad to see that Default Folder X still works perfectly in the new OS. Which brings me to my question for all of you: What's your must-download Mac program? Let me know in the comments!