Here's a short list of LaTeX and HTML commands for producing the symbols often used when writing up papers or making scientific webpages.

Symbol LaTeX Command HTML Command Result*
Ångström \AA Å Å
Approximately equal to $\approx$ ≈
Not equal to $\neq$ ≠
Percent \% % %
Plus or minus $\pm$ ± ±
Times $\times$ × ×
Scalar multiplied by $\cdot$ · ·
Divided by $\div$ ÷ ÷
Less than $<$ &lt; <
Less than or equal to $\leq$ &le;
Less than or similar to $\lesssim$
Greater than $>$ &gt; >
Greater than or equal to $\geq$ &ge;
Greater than or similar to $\gtrsim$
Square root $\sqrt{...}$ &radic;
Squared $^2$ &sup2; ²
Cubed $^3$ &sup3; ³
Degrees $^\circ$ &deg; °
Micro- $\mu$ &micro; µ
Of the order of (similar to) $\sim$ &sim;
Prime $'$ or $\prime$ &prime;
Double-prime $''$ (two single quote-marks) &Prime;

* As rendered in your browser.