Online pdflatex compiler that allow you to do a collaborative LaTex document.
A great way of finding LaTeX symbols just drawing it.
Documentation about TikZ.
Live equation editing · permanent short links · LaTeX+AMS input.
Write an equation and get the LaTex or MathML source.
An open source JavaScript display engine for mathematics that works in all modern browsers.

The following are examples of MathJax:

Complex formula:

\[\begin{align*} & \phi(x,y) = \phi \left(\sum_{i=1}^n x_ie_i, \sum_{j=1}^n y_je_j \right) = \sum_{i=1}^n \sum_{j=1}^n x_i y_j \phi(e_i, e_j) = \\ & (x_1, \ldots, x_n) \left( \begin{array}{ccc} \phi(e_1, e_1) & \cdots & \phi(e_1, e_n) \\ \vdots & \ddots & \vdots \\ \phi(e_n, e_1) & \cdots & \phi(e_n, e_n) \end{array} \right) \left( \begin{array}{c} y_1 \\ \vdots \\ y_n \end{array} \right) \end{align*}\]

The following is a math block:

Inline \(5 + 5\) $$ 5 + 5 $$

But next comes a paragraph with an inline math statement:

\(5 + 5\)