Algebra

AlgebraHelp.com - Offers free lessons in Algebra, covers all basic topics such as FOIL, Simplifying polynomials substitution, etc.

Algebra Modules at  Purplemath.com - An excellent site with notes and explanations on almost all algebra topics, from the simple distributive property to induction proofs, this site has it all.

Algebra Links at  Purplemath.com - From those who brought you the Algebra Modules, comes perhaps the greatest collection of Algebra links ever created...

MathGoodies - Contains "interactive math lessons" on common Algebra topics.  The layout seems a bit childish...

Algebra Online - Contains message boards, equation solves, inequality solvers, and more.

Geometry.net - Basically a math search engine, it finds articles relating to Algebra and Geometry.  Although the front page contains categories, they are really just searches of those topics.

Geometry

The Geometry JunkYard - Don't let the name fool you, this place has got some good junk.  Covers basic - intermediate level topics.

Geometry Formulas and Facts - This site covers almost all levels of geometry.  However, it is not sorted by difficulty level, but by chapters in some college math book.  Just scroll down until you find the topic that you're looking for, and don't worry about it if you don't know what every topic above it means, because if you did, you wouldn't be taking geometry at SPHS...

Geometry at ThinkQuest - Offers notes and examples on basic geometry topics.

Differential Geometry - Notes pertaining to differential geometry.   Will you learn this stuff in high school?  Heck no.  However, it may come in handy for an extra credit report or something...

Geometry at Yahoo - Yahoo category containing links pertaining to geometry.

Calculus

Single Variable Calculus Notes - My notes for high school calculus, equivalent to the AP Calculus AB test. Based on UCSD Math 20A/20B. It covers differentials and integrals w/ respect to 1 variable.

Multivariable Calculus Notes - My notes for high school/college calculus, equivalent to the AP Calculus BC test. Based on UCSD Math 21C. It covers partial derivatives, double and triple integrals, vector functions/curves, parametric/polar equations, sphereical/cylindrical coordinates, and quadric surfaces.

Calculus@Internet - Covers Pre Calculus, a wide range of Calculus topics, Applications, and much more.  An excellent place to start if you're looking for Calculus information.

The Calculus Tutor - Covers basic calculus, as well as some Pre Calculus

The Integrator - Input any integral, and watch it solve it for you.

AP Calculus Test Preparation - Created by some high school to help students prepare for the AP Calculus test.  Includes sample tests, sample AP questions, and other resources

Other Math

Sequences and Series - My notes for Sequences and Series, based on UCSD Math 21D. Chances are you won't be learning this in high school, but you might in college if you're taking science/engineering. It covers normal sequences/series, alternating series, Taylor/MacLauren series, and Taylor inequalities

Differential Equations, Page #2 - Laplace Transforms - My notes for Differential Equations, based on UCSD Math 21D. Chances are you won't be learning this in high school, but you might in college if you're taking science/engineering. It covers first and second order DE's.

Geometry in Action - Practical Applications of Geometry, sorted by field and profession.

Applications at Calculus@Internet - Practical Applications of common Calculus topics such as derivatives, integrals, etc.  Also gives specific applications in Science, Engineering, and Business.

AskJeeves.com - A different type of website.  AskJeeves is a large message board.  You can post your message, and given a few days, someone is bound to respond...Hopefully...

Practical Uses of Math and Science - The name says it all...  Provided by JPL.

Online Math Applications - Contains some basic math applications that we never even dream of.  Some are a little childish, but they're still applications.

Math Applications - A collection of common online calculators and games such as Mortgage loan payment calculators and calorie consumption calculators.  If you can figure out how these things work and the equation behind them, they make excellent real world applications of algebra...

Ask Dr. Math - Similiar to AskJeeves.  While AskJeeves gives you a larger audience, the audience is more general.  Ask Dr. Math has a very limited audience, but everyone who goes is at least somewhat interested in math, hence would probably have more knowledge than the general public.  Quantity or quality, it's your call.