What check does the president have on Congress?

Checks and balances, the idea of making sure no one branch of government has complete or unchecked power, was an idea written by James Madison in the Federalist Papers, essay number 51.  Checks and balances also call for a separation of powers within government.
The president, head of the executive branch of government, can check the legislature, just as the legislative branch can check the president.  When a bill comes up in Congress, it needs to be passed by both houses in the Capitol.  If legislation is passed through both the House of Representatives and the Senate, the president has the power to veto this legislation, essentially shutting it down.  Congress can override the presidential veto with a two-thirds vote.
Another check the president has on Congress is the ability to call a special session, propose executive orders, and reject portions of bills (instead of the whole bill); while some of these are considered formal or informal presidential powers, they still count as a mechanism that can check the power of Congress.