Initially, Leopard Finder has 6 pre-defined smart folders on it sidebar. Actually, there are 11 of them provided. How to activate all 11 of them?

They are called CannedSearches

First of all, you should spot them first. If you follow these simple steps, spotting them won't be hard :

1. Open Finder
2. Go to Macintosh HD ▸ System ▸ Library ▸ CoreServices ▸ Finder
3. Right-click on Finder icon and choose Show Package Contents
4. Dig inside Contents ▸ Resources ▸ CannedSearches

Plug Them All

Here you should be able to see 11 predefined search items. I believe, you won't be patient enough to read ahead and start dragging them to your sidebar. And you find out that those items cannot be dragged directly to sidebar.

But sure, there is a way to place them nicely on your Finder sidebar :

1. Choose one of them
2. Right-click and then choose Show Package Contents
3. Copy the folder named search.savedSearch to your Desktop (you can also copy it to your home folder)
4. Rename it
5. Now you can drop it to your Finder sidebar and the file you copied just now can be deleted

I'd like to thanks Apple Matters and the author Tanner Godarzi for this nice trick.