Somewhat related to smell, here's a fun chemical that you might not have heard about: thioacetone. Thioacetone (and some other organosulfur compounds) are extremely malodorous. During distillation of this chemical in the German city of Freiburg, it resulted in vomiting, nausea and unconsciousness within a radius of 0.75km of the laboratory. It's also reported to smell worse when diluted and the odor is referred to as "fearful".[0]

[0] http://blogs.sciencemag.org/pipeline/archives/2009/06/11/thi...

Check out other works in Derek Lowe's Things I Won't Work With series for other similarly dangerous chemicals. I first learned of thioacetone from him and have seen other articles of his floating around HN as well.