Newlib is a C library intended for use on embedded systems. It is a conglomeration of several library parts, all under free software licenses that make them easily usable on embedded products.
Newlib is only available in source form. It can be compiled for a wide array of processors, and will usually work on any architecture with the addition of a few low-level routines.
Newlib is the C Library used by Cygwin and RTEMS.