NIST- USA government standardization agency. Good for boiling point, thermodynamic data, etc.
Redox potentials- It's wikipedia, so it's pretty trustworthy, but not perfect.
Semiconductor Bandgaps- Again, its wikipedia.
Acid Dillutions- A lazy man's way to calculate how to dillute acid to 1M.
Q
Energy information
Energy Institue's Statistical Review of World Energy- (Used to be run by BP oil) Overall summary of world energy supply and demand
EIA energy report- Overall summary of US and world energy. Data is similar to BP's report.
NordPool- Day ahead wholesale electricity prices for most of Europe
Intercontinental Exchange (ICE)- European natural gas prices from the port of Rotterdam.
Aggregated Gas Storage Inventory (AGSI)- Natural gas storage levels throughout Europe
Energinet.dk - The Danish national transmission operator. Almost all of Danish energy data can be found here.