Main entry points on the Web :

official/private crystallography sites :
 

IUCr: http://www.iucr.org/iucr-top/welcome.html
Content
What's new page
Databases
Teaching
Sincris
Sincris full software list
Crystallographic servers
Forthcoming conferences
Radiation resources
Proceedings on past meetings
Journals
Search in IUCr Journals
Sources of News
etc
 
 

More specifically dedicated to powder diffraction :

CCP14 :  http://www.ccp14.ac.uk/
 
 
 
 
 
 
 
 
 
 
 
 

More specifically dedicated to SDPD

SDPD-Database : http://www.cristal.org/iniref.html
 
 
 
 
 
 
 
 
 
 
 
 

Search engines :

Altavista, Lycos, Hotbot,

etc