Mathematics

Tutorials and documentation concerning mathematics with LaTeX:

• Mathmode
  by Herbert Voss
  Extensive collection of materials concerning mathematics with TeX and LaTeX, highly recommended
• amsmath user’s guide
  American Mathematical Society
  Documentation for the math package
• Math into LaTeX
  by George Grätzer
• Short Math Guide for LaTeX
  by Michael Downes
  concise summary of recommended features in LaTeX and extension packages for writing math formulas
• Using the amsthm Package
  American Mathematical Society
  enhancements of the LateX theorem environment
• Höhere Mathematik
  chapter 8 of the german edition of the LaTeX Companion by Mittelbach, Gossens
• Mathematik mit LaTeX
  easy introduction by Jana Peters

Useful packages for writing mathematics:

• amsmath – the most important package for mathematics with LaTeX, fundamental for complex math documents
• amsthm – environments for theorems, definitions and proofs
• ntheorem – alternative theorem package, improved handling of endmarks
• empheq – emphasizing formulas by color, frames or symbols
• mathtools – some useful math extensions
• cancel – strikeout expressions and formulas
• amscd – commutative diagrams with amsmath
• xypic – for construction of math diagrams

Parts of this lists were translated and used here for the goLaTeX wiki. and in the TeX.SX mathmode tag wiki