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By default you only get one, and you can select Recent Applications, Recent Documents, Recent Servers, Favorite Volumes or Favorite Items. Leopard forces you to pick one. Well, I wanted more than one, so I ran the following command in Terminal:

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

And a new Recent Items stack appears. Just right click and set it to whatever you want it to be. To delete it, just drag it off the dock.