Folly is a large collection of reusable C++ library components that internally at Facebook are used extensively. While this may not be too interesting for end-users, the released C++ code may prove to be useful since Folly was designed to be extremely fast and fulfill Facebook's performance requirement needs.
Besides being speedy, the Folly code was also designed to be easy to use and highly reusable as it's worked on by hundreds of Facebook engineers. Facebook claims some of these utilities/libraries are easier to use than other available alternatives at this point. Folly is also complementary to other libraries like Boost and the C++ Standard Library rather than suffering from the "Not Invented Here" syndrome that plagues some companies.
More information on the Folly open-source library is available from the news item at Facebook.com while the code can be downloaded from GitHub. This code is available under the Apache 2.0 license.