The International Tables for Crystallography from IUCr is now available online. The full set of all the volumes from Volume A to G including A1 is available.
Volume A Space-group symmetry
Volume A1 Symmetry relations between space groups
Volume B Reciprocal space
Volume C Mathematical, physical and chemical tables
Volume D Physical properties of crystals
Volume E Subperiodic groups
Volume F Crystallography of biological macromolecules
Volume G Definition and exchange of crystallographic data
A guided tour is available, which contains five sample chapters and numerous examples showing the functionality of the site.