Chemistry
Electrochemistry Dictionary
http://electrochem.cwru.edu/ed/dict.htm#index
In the Pipeline
http://pipeline.corante.com/
IUPAC Medchem Dictionary
http://www.chem.qmul.ac.uk/iupac/medchem/
NCBI PubChem
http://pubchem.ncbi.nlm.nih.gov/search/search.cgi
Organic Chemistry Portal
http://www.organic-chemistry.org/
Spectral Database for Organic Compounds
http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi?lang=eng
Totally Synthetic
http://www.totallysynthetic.com/blog/