I've worked with numerous projects using Typekit or Google Fonts, and a few that used typography.com by Hoefler & Co. Google Fonts has far and away been the best experience, both for development and for the users. Kudos to the Google Fonts team.

I wonder if Google plans to add a paid collection of premium fonts to its library – while I prefer open source fonts, I know a number of designers who use very specific fonts outside of what's currently available for free.