Thomas Jefferson did one very significant thing
that really brought up a lot of questions over what the power of the President was.
And, to provide a more modern example:President Bush's warrantless wiretapping programs that we've been hearing a lot of brouhaha about lately is another good example of a "loose" interpretation of the constitution. Nowhere in the Constitution is the executive branch explicitly given permission to do what amounts to a search without a warrant; however, there might be some leeway for the executive if it relates to foreign policy, which the administration claims that it does. Do the immediate needs of fighting terrorism override the civil liberty concerns encoded in the Bill of Rights? A strict reading of the Constitution and the concept of separation of powers says no. But then again, look at what Jefferson's somewhat out-of-bounds actions brought us.
(I hope that I have done a decent job of leaving my personal opinions out of this.)