This file contains a collection of components for RepRap electronics. Place the RepRap.lbr file in the Eagle\lbr directory.
See Useful Software Packages#CAD software for electronics for other software for electronics.
You can find the current version of "official" reprap.lbr file at Reprap GitHub
Here are some supplemental vitamins. It was created because there was a need for the Pololu stepper driver board, and a shield for Arduino Mega.
Feel free to add any components that are used in RepRap electronics.
The Open Circuits: PCB footprints page lists a few more libraries in Eagle format, gEDA format, etc.