Chemistry

IUPAC Nomenclature of Organic Chemistry
Jmol

Physics

Usenet Physics FAQ - a very interesting collection of puzzles in different subjects of physics
How Everything Works - by Lois Bloomfield
The Flying Circus of Physics

General

Mensa - Are you mensa-worthy, i.e. among the top 2% IQ people? Only one way to find out --try.

Math and its applications

Online integrator- from Wolfram Mathematica
Wolframtones - instantly create unique music