One of the familiar complaints that NASA receives when its budget comes up for approval is that "...the money really ought to be spent down here instead of up there". Leaving aside the fact that NASA's civil servants and contractors all live here on Earth, and thus the money is spent here, NASA's fifty years of research and development have resulted in a wide range of inventions and processes, ranging from the complexity of image processing through the simplicity of fire-resistant kid's pyjamas.

http://www.hq.nasa.gov/office/hqlibrary/pathfinders/spinoff.htm

The less obvious and most important benefit is spinoff technologies. The simple fact is that every new step we make in space exploration advances our knowledge of not just the Universe but the new height human innovation and technology can achieve. Some scientists have already hypothesized that if a civilization from another part of space were to make first contact with Earth their technology would be several orders of magnitude more advanced than ours because the many scientific and technological milestones they would need to achieve to make the feat even possible.