Crystallography related websites
APS Powder Diffraction Crystallography Resources
Elves manual - X-ray Basics

CCP14 Homepage Crystallography CCP14
CCP14 Tutorial List CCP14
GSAS VRML CCP14
GSAS Powder Fourier Map CCP14
Setting up POLA or IPOLA and POLA correctly for a powder XRD system containing a diffracted beam Graphite Monochromator in GSAS CCP14

International Union of Crystallography
(IUCr) Structure Reports help with preparing CIFs
World Directory of Crystallographers
ICDD.com

Miller Indices Explanations with diagrams.
Determination of Reduced Cells in Crystallography
Powder diffractometers performances as estimated by the Rietveld method
Atomic Scattering Factors
Neutron scattering lengths and cross sections
Anomalous Dispersion (fprime) Calculations
Crystallography lab Univ. Oklahoma
PCG Rietveld School Tutorials - Y2O3 - Fullprof
Theoretical Density

Rietveld
Fullprof Tutorial
Rietveld R and fit factors
Combined X-ray Neutron Rietveld
Structure Determination by Powder Diffractometry Course

Books
Springer - Modern Crystallography 2 - by Boris K. Vainshtein, et al.
Worked examples in the Geometry of Crystals, the 2nd edition (Univ. Cambridge)

Databases
ICSD for WWW
Mineralogy DatabaseInformation on the structures of minerals/solid state compounds.
The American Mineralogist Crystal Structure Database contains cif files of many crystal structures
CCDC- Cambridge Crystallographic Data Centre
Powbase - Powder Patterns Database cifs
Open Access Crystallography
SpringerMaterials Landolt-Bornstein

Symmetry
Space Group Frequency at Texas A&M
Bilbao Crystallographic Space Group Symmetry Tables
Group-Subgroup relationships
Symmetry Tutorial Group theory
Superspace Groups
(3+1) dimensional superspace groups

Equation for Bond distance in any crystal system
d2 = (x1 - x2) 2a2 + (y1-y2)2b2 + (z1-z2)2c2 + 2(x1-x2)(y1-y2)ab cos g + 2(y1-y2)(z1-z2)bc cos a +2(z1-z2)(x1-x2)ca cos b