Detexify
The easiest way to search for a math symbol is Detexify. This webservices allows you to draw the symbol. It looks like this:
Symbol tables
Arrows
| $\rightarrow$ | \rightarrow | $\leftarrow$ | \leftarrow | $\Rightarrow$ | \Rightarrow | $\Leftarrow$ | \Leftarrow |
| $\leftrightarrow$ | \leftrightarrow | $\Leftrightarrow$ | \Leftrightarrow | $\nRightarrow$ | \nRightarrow | $\nrightarrow$ | \nrightarrow |
| $\leadsto$ | \leadsto | $\mapsto$ | \mapsto | . | . | . | . |
Greek
| $\alpha$ | \alpha | $\beta$ | \beta | $\gamma$ | \gamma | $\delta$ | \delta |
| $\zeta$ | \zeta | $\eta$ | \eta | $\theta$ | \theta | $\epsilon, \varepsilon$ | \epsilon, \varepsilon |
| $\iota$ | \iota | $\kappa$ | \kappa | $\lambda$ | \lambda | $\mu$ | \mu |
| $\nu$ | \nu | $\xi$ | \xi | o | o | $\pi$ | \pi |
| $\rho$ | \rho | $\sigma$ | \sigma | $\tau$ | \tau | $\upsilon$ | \upsilon |
| $\phi$ | \phi | $\chi$ | \chi | $\psi$ | \psi | $\omega, \Omega$ | \omega, \Omega |
| $\Phi$ | \Phi | $\varphi$ | \varphi | . | . | . | . |
| $\Lambda$ | \Lambda | $\Delta$ | \Delta | . | . | . | . |
Operations
| $\cdot$ | \cdot | $\oplus$ | \oplus | $\times$ | \times | $\nabla$ | \nabla |
| $\pm$ | \pm | $\mp$ | \mp | $\cup$ | \cup | $\cap$ | \cap |
Relations
| $\approx$ | \approx | $\sim$ | \sim | $\cong$ | \cong | $\neq$ | \neq |
Calligraphic Letters
| $\cal{O}$ | \cal{O} | $\mathfrak{M}$ | \mathfrak{O} | $\mathfrak{R}$ | \mathfrak{R} |
Sets
| $\mathbb{N}$ | \mathbb{N} | $\mathbb{Z}$ | \mathbb{Z} | $\mathbb{R}$ | \mathbb{R} | $\mathbb{C}$ | \mathbb{C} |
| $\mathbb{A}$ | \mathbb{A} | $\cap$ | \cap | $\cup$ | \cup | $\in$ | \in |
| $\subseteq$ | \subseteq | $\subsetneq$ | \subsetneq | $\notin$ | \notin | $\bigcup$ | \bigcup |
Other
You might need \qed, \qedsymbol, \blacksquare for proofs. It is the Tombstone $\blacksquare$
I've recently needed $\dots, \ddots, \vdots$ (\dots, \ddots, \vdots) for a visualization in a matrix. Note that you can write ... instead of \dots, but you'll lose the semantics.
See also
- List of symbols - 164 pages of symbols
- Displaying a formula