What powers that were reserved for Congress are now in the hands of the President?

I would say that the major power that was meant to reside
with Congress, but has been taken by the Presidency, is the power to declare
war.

Although the president is the commander-in-chief of
the armed forces, the Constitution says that it is the role of the Congress to decide
when America will go to war.  This is, for the most part, no longer how things are
done.  Presidents have claimed the power to send troops to areas where they will engage
in combat without having Congress declare war first.  In fact, Congress has not declared
war since 1941 and yet the US has fought many wars since
then.

By taking the power to commit troops to combat, the
Presidency has taken a power that was supposed to be in the hands of
Congress.