Newlib is a collection of software from several sources, and was put together by Cygnus for embedded systems. It contains the routines required by the C Standard as well as a math library and misc. Newlib is a collection of software from several sources, each with their own copyrights, but basically it's a Berkeley style copyright. Resource Utilization Glibc, being intended for native Unix environments, does not need to worry about memory usage as much. GCC now comes with generic software floating point support for almost all embedded targets.
Several copies exist so that the correct one can be used when a given option is used.
For example, when compiling for the m68000, you do not want to link in a library compiled for the m68020. And so on. This varies over time so listing them here might not be useful. The best way to find out is to study the config-ml.in script at the top of the newlib source tree.