There are several non-alphabetic symbols, the best known is the integral sign $\int$
and the Weierstrass $P$-function $\wp$. To be sure their origins are letters of Latin alphabet, but they are special stylized symbols, and as far as I know there is no computer code for them in the standard sets of computer characters. Strictly speaking they do not belong to any alphabet. $\wp$ imitates Weierstrass handwriting. Symbols $\partial$ for the partial derivative and $\infty$ also belong to this list.
There are also mathematicians who use Cyrillic letters, for example John Milnor used Л
for some standard function is hyperbolic geometry, but this can hardly be called
"widely accepted". Л is the first letter in Lobachevski's name (Лобачевский).
I've seen other Cyrillic letters used, but again this is not "widely accepted".
An interesting case is the "letter" $\nabla$ usually used for the gradient. This is a apparently not a letter from any alphabet, and it is called "Nabla" which is the Greek word for some Phoenician musical instrument. See Nabla symbol. Unlike $\wp$ it is included in Unicode.