Google web fonts has been dominating the free web fonts space for a while now, and rightly so, they have over 600 fonts and some of them are absolutely fantastic.
Adobe Edge Web Fonts has actually existed since May but it hasn't seem to have got much traction until now. The service first of all offers a much better preview interface for the fonts compared to Google web fonts. You can search for fonts in all sorts of ways and look for specific types of fonts.
Some of the fonts you'll already find on Google Web fonts, but there are some others you won't find anywhere else. You don't need to sign up to use fonts either.