According to the Typekit blog, the newest open source font from Adobe, Source Serif Pro, is available via Typekit. I love the entire Source family (Sans Pro, Code Pro, and now Serif Pro) and it’s what I use on this site.
The blurb on Typekit does a great job of describing the font
A fresh take on the transitional style, Source Serif was designed as a companion to Source Sans. Their close relationship was achieved through a careful match of letter proportions and typographic color. Designed for a digital environment, the letter shapes for Source Serif are simplified and highly readable. The historical background, in combination with a high technical standard, gives the typeface a strong character of its own, lending itself to setting extended text on both paper and screen.
Currently there isn’t an italic version, but it’s in the works.
In addition to using it on Typekit, you can keep tabs on it over on Github.