Chem Spider

ChemSpider is a project of the Royal Society of Chemistry (the older, British cousin of ACS) that provides free access to information about over 25 million chemicals. Need to know what BHT stands for, or the boiling point of dimethyl sulfoxide? ChemSpider provides this and lots more: 2- and 3-D structure images, some spectra, references from the literature, safety information, etc. It also has a synthetic section that provides lab suggestions / tips / warnings often missing for primary literature.  There's no registration required to use it, and it links to all sorts of reliable sources for safety, physical properties, synthesis and toxicology.