UNITS If you get as confused about pascals, newtons, etc. as I do, have a look here.

PHYSICS  A-Level Physics covers most of classical physics.
                  The ABC's of Nuclear Science covers, at an elementary level, the main topics of nuclear physics.

 INORGANIC CHEMISTRY covers most areas of inorganic chemistry.

LIFE CHEMISTRY  the Life Chemistry paper in the Universe Story topic includes some good links.

BIOLOGY  TEXTBOOK covers most aspects of biology at a level suitable for the general reader.
Also try Kimball's Biology Pages for a very wide-ranging site.
This is a comprehensive account of biological evolution.

EARTH SCIENCES a very wide range of materials
But also have a look at
           Geology – UCMP Exhibit and West's Geology Resources are worth looking at if only for the
             County Maps
           METEOROLOGY a good introduction to the subject.
           The Remarkable Ocean World

ASTRONOMY There is a rather nice digital orrery  and a comprehensive history site 

And of course there is always wikipedia