The AST library provides a comprehensive range of facilities for attaching world coordinate systems to astronomical data, for retrieving and interpreting that information and for generating graphical output based on it. It's main selling points are:

Ease of use.
Facilities for generating plots of generalised non-linear, potentially discontinuous 2-D or 3-D coordinate systems, with detailed control of the appearance of the plot.
Facilities for converting transparently between different coordinate systems, including a wide range of celestial, spectral and time coordinate systems.
Facilities for searching a general collection of connected coordinate systems for a coordinate system with any given set of characteristics.
Allows code for handling WCS information to be written in a general way without regard to the specific nature of the coordinate systems being handled (i.e. whether they represent sky positions, spectral positions, focal plane positions, pixel positions, etc).
Flexible system for saving and retrieving WCS information, including (but not limited to) a range of different popular FITS descriptions.
Written in C but has interfaces for C, Fortran, Python, Java (via JNI), Perl, and UNIX shell.
Extensive documentation.
LGPL licence.