The implementation of pbrt-v3, the version of the system for the third edition of the book (released in 2016) is available from github. (If you don't want to use git, the latest source code is available as a zip file.) See the User's Guide for information about building this version of the system.
Over 10GB of interesting scenes to render are available for use with pbrt-v3.
Now and again, we write up literate programs describing implementations of additional rendering algorithms and make them available for download. (We expect that many of these will end up as content in a future fourth edition of the book.)
For now, we have just The Implementation of a Hair Scattering Model.
Two chapters of earlier editions of the book are available for download: you can download both Chapter 7, Sampling and Reconstruction, from the first edition of the book as well as Chapter 4, Primitives and Intersection Acceleration from the second edition.
The implementation of pbrt-v2, the version of the system that accompanies the second edition of the book (from 2010), is also available from github. See the page on building pbrt-v2 for instructions for how to build pbrt on your system. A number of interesting scenes are also available to use with pbrt-v2.
The source code for the version of pbrt that accompanied the first edition of the book (in 2004) is also available from github. It's only useful for historical purposes at this point; the version of the system that shipped with the third edition of the book has numerous improvements.