Apple - How can I create a keyboard shortcut to access a specific folder in Finder?

One option would be to create an Automator service and assign it a shortcut in the Keyboard preferences.

tell application "Finder"
    reopen
    activate
    set p to (system attribute "HOME") & "/Downloads/"
    set target of Finder window 1 to POSIX file p
end tell

You could also save the script to ~/Library/Scripts/Applications/Finder/ with AppleScript Editor and use FastScripts to assign the shortcut.

Solution using Quicksilver

That's it.

As an alternative to Quicksilver, consider Launchbar. It is a religious preference, so rather than discuss why to choose one over the other, I just wanted to make you aware of both.

Another commercial gem is Keyboard Maestro, which is very much overkill for this sort of job (Automator is the simplest approach, as described by @Lri and @gentmatt), but if you are the sort of person who likes keyboard shortcuts to folders, you will probably find a lot to love in Keyboard Maestro.