Some of the commonly used symbols:
infty - Infinity
leq - Less then or equal
geq - Greater then or equal
partial - Partial differential
sum - Summa
prod - Product Sign
subset - Contained in
in - Element of
cup - Union (if you want to see big symbol, enter igcup
eq - Not equal to
approx - Almost equal to (asymptotic to)
equiv - Identical to (equivalent)
int - Integral
oint - Contour integral