PlanetMath - Math for the people, by the people

A vibrant mathematical community with 5 "General Questions" forums catering for all academic levels from High School to Research. About 4000 questions have been asked in the University/Tertiary forum since 2002 - many have been answered, sometimes provoking lengthy discussions.

"PlanetMath is a virtual community which aims to help make mathematical knowledge more accessible. PlanetMath's content is created collaboratively: the main feature is the mathematics encyclopedia with entries written and reviewed by members. The entries are contributed under the terms of the GNU Free Documentation License (FDL) in order to preserve the rights of both the authors and readers in a sensible way."

Mathematics and Statistics forum

From the Open University's OpenLearn LearningSpace, this is a brave attempt to establish an open and free maths forum where anyone can ask or answer questions. Unfortunately, many questions remain unanswered but there are a moderate number of contributors and some interesting discussions so please consider contributing!

"This is the place to discuss issues around the range and type of educational resources within the Mathematics and Statistics section of the LearningSpace. Anyone who's logged in can post to this forum."

MathNerds

A non-profit corporation run by unpaid volunteers providing a free service with hints, direction, references, or guidance (not necessarily answers) in many areas of mathematics. About 100 active volunteers respond to about 1500 questions a month with a remarkable 97% of all questions answered within 16 hours! You do have to register to submit questions and access the archives but this is straightforward. Many of the questions are from kids struggling with homework but it's still an excellent resource for the basics including some college level material.

The Math Forum

Drexel University's online resource for improving math learning, teaching and communication, offering problems and puzzles, online mentoring, research, team problem solving, collaborations and professional development. This is a more commercially orientated site so not everything is freely available but Ask Dr. Math does have a College Level Archive that you can freely browse questions and answers on:

Algorithms, Algebra, Analysis, Calculus, Definitions, Discrete Maths, Exponents, Geometry, Imaginary/Complex Numbers, Logic/Set Theory, Number Theory, Physics, Probability, Statistics, Trigonometry.