GDIS is a convenient periodic structure builder and visualizer by Sean Fleming.

From the webpage: "GDIS is a scientific visualization program for the display, manipulation, and analysis of isolated molecules and periodic structures. It is in development, but is nonetheless quite functional. It has the following features: Support for many common file formats (CIF, PDB, XTL, XYZ, and many more). Job submission tools for computational chemistry packages: GAMESS, GULP, ReaxMD, and SIESTA. Job analysis and graphing tools for dynamics simulations. Useful manipulation tools, including matrix transformations and supercell construction. Powerful surface generation and crystal morphology tools."