Are there notable organic compunds which look very similar but have very different properties?
One example of disastrously different effects of very similar molecules is thalidomide. There are two isomers, only differing in their stereochemistry. The (R)-isomer is responsible for the sedating effect it was initially used for. The (S)-isomer lead to horrifying birth defects in the children of woman that took that drug. 



It is somewhat more complicated, as the isomers are racemized quickly in vivo, so even if you administer only the (S)-isomer, it will still be converted to the (R)-isomer in the body.