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 \bigcup
\neq - Not equal to
\approx - Almost equal to (asymptotic to)
\equiv - Identical to (equivalent)
\int - Integral
\oint - Contour integral