I'm trying to find an easy way to get them all.... A quick search turned up this linux script which uses Mercurial.
http://www.webupd8.org/2011/01/automatically-install-all-goo...
http://www.webupd8.org/2011/01/automatically-install-all-goo...
Google changed repository layout so there are a lot more subfolders to sort through but `hg clone https://code.google.com/p/googlefontdirectory/` still works fine.
(edit: just saw @shadowmint's update, agreed about https://github.com/w0ng/googlefontdirectory being better in every way.)
https://code.google.com/p/googlefontdirectory/#Download_Sour...