Senator John McCain has been telling people that Barack Obama requested $3 million for an overhead projector for the Adler planetarium in Chicago as though this was a bad thing and a waste of taxpayers money. He hasn't just said this once, he's said it several times, most recently in the 2nd presidential debate.
Now, I'm not sure who feeds him this information but it is obvious that when he uses the term "overhead projector" he may be thinking of that thing you saw in classrooms and boardrooms years ago. $3 million would be a large amount of money for such a piece of equipment.
However, he is wrong, misleading and/or intentionally deceitful. Let me show you why:
This is an overhead projector:

It costs about $250-$400. It is an outdated piece of equipment that has been replaced by laptops and Powerpoint. It is usually used by businessmen trying to show why they need more money.
This is a Zeiss Planetarium projector:

It costs several million dollars and is used to educate children and adults about the universe they live in. This is what Barack Obama requested the 3 million dollars for. Don't you think the oldest planetarium in the western hemisphere and a source of great science education for the population of America deserves to have up-to-date equipment?
The Adler and planetariums like it have taught literally millions of people the wonders of the universe. Children have embarked on careers in astronomy and space science as a result of their trips to a planetarium.
There are a thousands of places where far more than $3 million dollars has been frittered away in useless earmarks and pork. Senator McCain has chosen probably the worst example of waste and has instead has highlighted an example where an earmark actually benefited the public and was possibly there because the funding wasn't available through regular channels.
America needs a president who is honest, intelligent and shows integrity. In the "overhead projector" charge John McCain shows he has none of these.