Google Web Fonts

The Google Web Font service is free to use, and has a fairly extensive library of open source fonts. You do have to test thoroughly to be sure that the fonts are hinted well and render nicely on all platforms, and there may not be as many weights and variants available, but there are some real gems worth exploring. I’ve used them extensively in these demos in order to make the code as portable as possible: Roboto, Roboto Slab, Merriweather, Open Sans, and Source Sans are all excellent typefaces that stand up well everywhere I’ve tested them.