Spglib is a C library to find and handle crystal symmetries.

Features
Find symmetry operations
Identify space-group type
Wyckoff position assignment
Refine crystal structure
Search irreducible k-points
Find a primitive cell

Algorithms
Space-group type finding algorithm: R. W. Grosse-Kunstleve, Acta Cryst., A55, 383-395 (1999)
Crystal refinement algorithm: R. W. Grosse-Kunstleve and P. D. Adams, Acta Cryst., A58, 60-65 (2002)

There is also a python module called pyspglib.

http://spglib.sourceforge.net/