Some websites that may come in handy to the average science student:

Physics / Physical Chemistry:

An animation of the Ideal Gas Law - http://www.7stones.com/Homepage/Publisher/Thermo1.html

Organic Chemistry:

Spectral Database for Organic Compounds - http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi

Heterocyclic compounds; naming and properties - http://www.cem.msu.edu/~reusch/VirtualText/heterocy.htm

General Chemistry:

Interactive Periodic Table of the Elements - http://www.dayah.com/periodic/

Chemical Safety Database - http://www.inchem.org/

Biology:

Interactive animation of mitosis - http://www.cellsalive.com/mitosis.htm

Biochemistry:

National Centre for Biotechnology Information - http://www.ncbi.nlm.nih.gov/