Although the molecule is a poisonous solid, as flesh decays the vapor pressure of the putrescine it contains becomes sufficiently large to allow its disgusting odor to be detected. It is usually accompanied by cadaverine (named after the cadavers that give rise to it), a poisonous syrupy liquid with an equally disgusting smell. Putrescine and cadaverine also contribute towards the smells of some living processes. Since they are both poisonous, the body normally excretes them in whatever way is quickest and most convenient. For example, the odor of bad breath and urine are 'enriched' by the presence of these molecules, as is the smell of semen, which also contains the related molecules spermine and spermidine.
some silly molecules here - http://www.bristol.edu/depts/Chemistry/MOTM/silly/sillymols.htm