Free Web-Fonts Service: Adobe Edge Web Fonts
Google Web Fonts is no doubt the most popular free web-fonts service out there.
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.