There is an astounding number of resources freely available! Rather than produce a large list (isn't that what Google is for?), here is a select, high-qualty list:

• Paul's Online Math Notes:
  http://tutorial.math.lamar.edu/
  Contains notes and formula sheets for Algebra/Pre-Calculus, Calculus I, II, and III (Differential, Integral, and Multi-variable), Linear Algebra, and Differential Equations.
  
  
• S.O.S. Math:
  http://www.sosmath.com/
  Notes and examples for Algebra, Trigonometry, Calculus I and II, Linear Algebra, and Differential Equations
  

• Hippocampus.org:
  http://www.hippocampus.org/Calculus
  http://www.hippocampus.org/Statistics
  http://www.hippocampus.org/Physics
  Contains audio lectures / lecture notes, accompanying slides, and interactive exercises.
  

• MIT OpenCourseWare: (courses with videotaped lectures)
  Calculus I (Differential)
  http://ocw.mit.edu/OcwWeb/Mathematics/18-01Fall-2006/CourseHome/index.htm
  Calculus III (Multi-variable)
  http://ocw.mit.edu/OcwWeb/Mathematics/18-02Fall-2007/CourseHome/index.htm
  Differential Equations
  http://ocw.mit.edu/OcwWeb/Mathematics/18-03Spring-2006/CourseHome/index.htm
  Algorithms and Complexity
  http://ocw.mit.edu/OcwWeb/Electrical-Engineering-and-Computer-Science/6-046JFall-2005/CourseHome/index.htm
  
  
• http://www.mathhelpforum.com/math-help/
  
  
• More textbooks:
• http://mecmath.net/ -- A free vector calculus textbook
• http://www.collegeopentextbooks.org/opentextbookcontent/open-textbooks-by-subject/math.html -- a list of free textbooks
• http://sites.google.com/site/whitmanmathematics/ -- A free Calculus 1-3 textbook
• http://www.khanacademy.org/ -- video examples of all types of mathematics
• http://linear.ups.edu/opentexts.html -- A free textbook for Linear Algebra
• http://artsci.drake.edu/grout/doku.php/teaching_resources -- A list of teaching materials
  

• Resources for advanced mathematics:
  
• Probability:
  Textbook: http://math.dartmouth.edu/~prob/prob/prob.pdf
  Website: http://www.dartmouth.edu/~chance/teaching_aids/books_articles/probability_book/book.html
• Interactive Real Analysis:
  http://web01.shu.edu/projects/reals/
• Analysis and Advanced Calculus:
  http://www.maths.abdn.ac.uk/~igc/tch/ma2001/index/index.html
• Analysis:
  http://www.math.louisville.edu/~lee/
• Analysis webnotes:
  http://www.math.unl.edu/~webnotes/contents/chapters.htm
• Abstract and Linear Algebra (Module Theory):
  http://www.math.miami.edu/~ec/book/
• Abstract Algebra:
  http://www.math.uiuc.edu/~r-ash/
• Algebraic Topology:
  http://www.math.cornell.edu/~hatcher/AT/ATpage.html
• Analytic Number Theory:
  http://www.mth.uea.ac.uk/~h090/antmain.pdf
• Graph Theory:
  http://www.math.uni-hamburg.de/home/diestel/books/graph.theory/download.html
• Advanced algebraic Topics:
  http://www.jmilne.org/math/CourseNotes/