Depending on your interests, one of the two most recent versions of pbrt is likely to be most suitable.
A variety of interesting scenes are available for rendering with pbrt.
There is a pbrt users mailing list that can be a helpful resource for information about the system. We also occasionally post pbrt-related announcements to this list.
Now and again, we write up literate programs describing implementations of additional rendering algorithms and make them available for download.
So far, we have:
A number of exporters are available to convert scenes from other 3D file formats to pbrt's format. The highly-capable assimp system is now able to export to the pbrt-v4 format, which provides an effective pipeline from FBX, OBJ, and numerous other formats into pbrt's.
Other exporters include:
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 full contents of the third edition are freely available online.
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 versions of the system that shipped with subsequent editions of the book have numerous improvements.