Google Web Fonts is no doubt the most popular free web-fonts service out there.
Adobe has launched an alternative named Adobe Edge Web Fonts where the fonts are served through Typekit.
The service includes the open source fonts in Google Web Fonts and some more.
Its usage is pretty straightforward, select the font and you get a .JS file to be included in your web pages. After that, use it as any other
font-family in your CSS rules.
Currently, there are ~500 font-families and a documentation exists for details on the usage.