Iucr.org is the online home of the International Union of Crystallography.
For those who might not be familiar with the term, Crystallography is the experimental science of determining the arrangement of atoms in solids. This union is dedicated to bringing together the people who work in this very interesting field of science. The Publications section is where you will find free online publications on the subject. These are a good way for both newcomers and professionals to get a feeling for what the research in this area is like. The publications can sound a little monotonous at times, but as you get more familiar with the subject, they should prove themselves more useful. The International Tables section is where you will find great resources for anyone that is just getting started with the study of the atom structures in solids. The tables provide great conversion patterns that might not be known to someone that just started studying the field. 
