Amidst the massive sea of information available on the internet there are a few notable gems out there which no chemist can live without. This is not at all to be exclusive, but below is just a few top picks.
http://www.onlineconversion.com/ - advertises that you can use the site to "Convert just about anything to anything else." especially useful for volume, density, energy, and other "hard-to-find" conversions.
http://scholar.google.com/schhp?hl=en - general resource for finding published peer-reviewed journal articles.
http://sdbs.riodb.aist.go.jp/sdbs/cgi-bin/cre_result.cgi?STSI=136078030756947 - great search engine for an enormous library of HNMR, C13NMR, FTIR, MS, and a wide array of other analytics information for common compounds
http://www.acdlabs.com/home/ - Has a variety of software packages for handling chemistry drawings and data.