Google Web Fonts is an awesome source of free, open source fonts hooked up to a world-class CDN. It's also, sadly, filled with one-off novelty fonts vying for the title of "the new Comic Sans."
I quickly discovered that a good metric for higher-quality fonts was the presence of four or more alternates. The Google Web Fonts directory does not allow that type of filtering, so I built this simple one-off page that allows you to browse typefaces that are true families.
These fonts are all downloaded. It should go without saying that the page will load slowly.