Open Sans is a free (for both commercial and personal use) open-source font designed by Steve Matteson for the Apache Foundation.
It includes Font-Weights 300 (Book), 400 (Normal), 600 (Semi-Bold), 700 (Bold), and 800 (Extra-Bold), as well as italic versions of all of the above.
We use Open Sans at 300 (Book) for basically the whole site, with bold and some smaller headers at 400. This gives us a clean and professional look. We will also be utilizing it in future videos and other sites, as well as some books.