Chemistry 
1. Chem Click
Directory with listings on chemical and chemistry technology related information.
http://www.chemclick.com/
2. ChemBuddy
Marcin Borkowski, Marki, Poland sells chemical calculators for pH calculation, concentration conversion, solution preparation, equation balancing, and stoichiometry with built in dissociation constants and density tables databases.
http://www.chembuddy.com/
Free searchable Periodic Table database with a description of all the elements.
http://www.elementsdatabase.com/