The Discovery space shuttle is about to make its decent from outer space, and I can't find a single, significant contribution that the shuttle has made that in any way justifies its price tag.

Clearly, space has certain national security implications.  The spy and reconnaissance satellites that we have in space are an ever increasing portion of our ability to gather intelligence on what is going on in the rest of the world.  But manned space flights are different.  Spending billions of dollars so we can learn what zero-g adds to geriatric science seems like a massive waste of money. 

But manned space flight has become the most visible face of NASA, so it seems like they're sending people into space just to keep the agency in the news.