Here are some websites that you might find useful.

Geometry-

California Math League
http://www.mathleague.com/help/geometry/polygons.htm#triangle
A description of geometric figures and polygons.

http://www.aaaknow.com/geo.htm
Geometric lessons on areas, perimeters, circumference, and other quantities.

http://www.math.com/homeworkhelp/Geometry.html
Information on polygons, 3-D figures, and theorems.

http://library.thinkquest.org/2647/geometry/gemometry.htm
Sample problems, formulas, glossary, and constructions.

Algebra

http://www.algebrahelp.com
Descriptions of many concepts from Algebra I & II

Yahoo! Education
http://education.yahoo.com
Homework help and Algebra message board

http://www.mathgoodies.com/students.html
Good stuff!

http://www.math.com/index.aspx
Good for all subjects

Pre-Calculus

http://www.themathpage.com/aPreCalc/permutations-combinations.htm
Permutations and Combinations

http://www.themathpage.com/aPreCalc/binomial-theorem.htm
Binomial theorem and Pascal's Triangle

Calculus

http://everything2.com/index.pl?node_id=939784
Tabular Method for Integration by Parts

http://archives.math.utk.edu/visual.calculus/4/substitutions.3/
Integration by Substitution

http://www.mccd.edu/faculty/powerd/M4b/M4b_L3_IBP.htm
Integration by Parts.

http://www.ma.utexas.edu/cgi-pub/kawasaki/plain/derivatives/2.html
Derivative calculator

http://www.math.com/homeworkhelp/Calculus.html
Integral rules and examples.

http://www.mathematicshelpcentral.com/lecture_notes/calculus_1.htm
A good place to review some of the major concepts of Calculus. Think AP test!