Common commands
Type this... ...to get this
x*y `x*y`
x/(y+z) `x/(y+z)`
dy/dx `dy/dx`
int_a^b f(x)\ dx `int_a^b f(x)\ dx`
(Delta alpha)/(Delta beta) `(Delta alpha)/(Delta beta)`
x^(y+z) `x^(y+z)`
sqrt x `sqrt x`
root3 x `root3 x`
sum_(n=1)^oo 1/n^2 `sum_(n=1)^oo 1/n^2`
prod_"p prime" (1 - 1/p^2) `prod_"p prime" (1 - 1/p^2)`
x in RR `x in RR`
cancel(2ab) `cancel(2ab)`
color(blue)1 + color(green)2 `color(blue)1 + color(green)2`
"text" `"text"`
"small"\ "space" `"small"\ "space"`
"medium" quad "space" `"medium" quad "space"`
"big" qquad "space" `"big" qquad "space"`

See asciimath.org for more commands.
There's more information about MathPaste here.