For a function to equal its inverse, we must have f(f(x)) = x. I'll give you one example:
f(x) = 1/x
If we evaluate f(f(x)), we get
f(f(x)) = f(1/x) = 1/(1/x) = x
Of course, this function is undefined at zero, but hopefully, that gives you enough of an idea to think of a few more.