Draw a two-dimensional crystal, by Nicolas Schoeni, Wes Hardaker and Gervais Chapuis: http://escher.epfl.ch/escher/
Tutorial on structure factors and maps in 2D, by Kevin Cowtan: http://www.ysbl.york.ac.uk/~cowtan/sfapplet/sfintro.html
1D Fourier transforms of a few atoms with selected B-factor, by Bernhard Rupp: http://ruppweb.dyndns.org/level1/new_cryscomp_applets.htm
Ewald sphere movie: http://lcr.epfl.ch/page37304.htm
Ewald sphere animation (program, need to download): http://bioc.rice.edu/Bioch/Phillips/Papers/XRayView/