There might be a post about these standards but here are some more interesting facts on the Helmet standards.
ECE (european Standard)
Differences between BSI 6658 Type A and SNELL
Differences between Snell M2005, DOT, BSI 6658-85 Type A and EN 22/05
Differences between Snell M2005, DOT, BSI 6658-85 Type A and EN 22/05 (another link with more info)
More information about DOT standard, HJC helmets SHOI's etc...
More detail on the testing methods and materials used.
edit: i'll be editing this post and adding additonal links