ShareLaTex:
Online pdflatex compiler that allow you to do a collaborative LaTex document.
Detaxify:
A great way of finding LaTeX symbols just drawing it.
TeXample.net:
mathURL:
Live equation editing · permanent short links · LaTeX+AMS input.
WebEquation:
Write an equation and get the LaTex or MathML source.
MathJax:
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:

$$5 + 5$$