Quicklook is the one reason I’ll never be satisfied on a non 10.5 computer ever again. The way it lets you look at any file with just a pop of the spacebar is something you just can’t imagine getting any better.. but then along comes The Folder Quick Look Plugin which you can find here.
Basically what it does is add a holy dose of extra goodies to Quicklook, including allowing you to view the contents of a folder without browsing into it at all. Check out Here

I find Leopards new Stacks feature incredibly useful, and fun to watch at the same time! Here are a few tweaks to help you make the most of stacks.
You can add a ” Recent Items” Stack to your dock by entering the following in terminal ( You can find terminal in /applications/utilities )
defaults write com.apple.dock persistent-others -array-add '{ "tile-data" = { "list-type" = 1; }; "tile-type" = "recents-tile"; }'
Once the stack appears you can right click it and select its type, and a few other features. To add a nice hover effect for the icons in a stack when expanded, Enter this line into the terminal
defaults write com.apple.dock mouse-over-hilte-stack -boolean YES
Type the same, with NO at the end to reverse this effect.
If you don’t like stacks and the reduced folder browsing capabilities, you can download Quay, Developed by Rainer Brockerhof, It Allows you to add your drive, or folders to your dock and browse them via a menu just as you would in OS X Tiger.

Alot of People don’t seem to like the new dock in OS X leopard, I however think it’s great, but needed a few changes, or at least a way to change it around and modify it.
I created this neat little piece of software called DockDR that enables you to do just that.
With a few clicks you can change the docks dimensions and even swap between 8 different dock skins without having to mess around in system folders or settings.
You can also Disable Dashboard and adjust Icons settings, Check it out at www.innermindmedia.com
You can find DockDr at www.innermindmedia.com , There is also a widget version that allows you to toggle between 3D and 2D dock styles.