In the Dock you can use the section to the right of the divider to store folders. Many people choose to put their user folder or Hard Drive here for easy access. One of the greatest benefits is that you can right click (Control click) on the icon to get a handy pop-up menu containing the contents of the folder. From here you can navigate through to any file contained within.

This works great, apart from with the Volumes folder. If you don't already know, the Volumes folder contains your Hard Drive, CDs, DVDs, Disk Images, iPods, iDisks and any other mounted media devices. To get to this folder, choose "Go To Folder" from the Go menu and enter /Volumes. You'll find that if you put this folder in the Dock you can still right-click to view the pop-up menu, however you can't navigate into any of the drives apart from your startup disk (Probably Macintosh HD).

The work around for this involves creating a folder action using AppleScript to make a self-updating Dock pop-up menu. Start by opening up Script Editor (located in Application/AppleScript). Paste the following script into the window that appears:

property scriptfolder : ((path to documents folder) as string) & "Drives"
on run
   tell application "Finder"
       if not (exists alias scriptfolder) then
           make folder at folder ((path to documents folder) as string) with properties {name:"Drives"}
       end if
   end tell
    Update()
end run
on adding folder items to thisfolder after receiving addeditems
   Update()
end adding folder items to
on removing folder items from thisfolder after losing theseitems
   Update()
end removing folder items from
on Update()
   tell application "Finder"
       set theFolder to folder script_folder
        delete every item of theFolder
        set theDisks to disks
        repeat with i in theDisks
           make new alias at theFolder to i
       end repeat
   end tell
end Update