Tools For Scientists:

Kaye & Laby online – free, searchable version of the current edition of “Tables of Physical and Chemical Constants”, first published in 1911:
http://www.kayelaby.npl.co.uk/

Molecular weight, nomenclature, etc:
http://webbook.nist.gov/chemistry/

Dictionary of units of measure:
http://www.ex.ac.uk/cimt/dictunit/dictunit.htm

Converting between units:
http://www.onlineconversion.com/
http://www.people.virginia.edu/~rmf8a/convert.html

Fundamental physical constants:
http://www.chemie.fu-berlin.de/chemistry/general/constants_en.html
http://physics.nist.gov/cuu/Constants/
http://www.iupac.org/reports/1993/homann/values6.html

Molecular weight calculator:
http://www.lmnoeng.com/molecule.htm

Periodic Table:
http://www.iupac.org/reports/periodic_table/
Online, interactive with options on displayed properties:
http://www.chemicalelements.com/index.html

Nomenclature - name of chemical from its structure:
http://www.iupac.org/nomenclature/ACD/calc_3dparty.html

Chemical sourcing search-engine, also contains info on physical properties of chemicals:
http://www.chemexper.com

Schools’ level science webpage:
http://www.science.ie

Science links websites:
http://www.chemdex.org/

Searchable materials' compatibility table, scientific glossary and unit-converter:
http://flw.com/datatools/