In Leopard you now have the ability to add several types of "recent/favorite items" to the Dock. To add a folder like this to the Dock, open up a terminal window and type the following and then press Enter:

defaults write com.apple.dock persistent-others -array-add '{ "tile-data" = { "list-type" = 1; }; "tile-type" = "recents-tile"; }'

Once that command completes, type the following command in your terminal window and press Enter:

killall Dock

Once you do this, you'll have a new Folder on your Dock. By default, this folder will show the Recent Applications. You can right click on this folder and select from the following options:

Recent Applications
Recent Documents
Recent Servers
Favorite Volumes
Favorite Items

But what if you want to have multiple of these folders so that you don't have to keep right-clicking to change what is shown? Turns out you can run the commands above multiple times and each time you run the command, a new folder will be added. So for example, you can run it three times so that you have separate folders for you Favorite Items, Recent Applications, and Recent Documents.

I find this to be very handy.