Signal vs. Noise is a weblog by celebrated designers and web app makers 37signals. Acta Display is used for headlines and Kepler for text. (On my Retina MacBook screen, the Kepler italic renders noticeably heavier than the roman. I wonder if others have the same experience.) Proxima Nova is the foundation typeface for 37signals’ main site and their flagship product Basecamp. It’s used in the header and footer navigation.