Laughing Wolf has some ideas.

Roles of a space agency:

Diversify life's portfolio. Earth is home to all known life in the universe. One uncharted asteroid could wipe out life on Earth. We need to put life on the moon, on Mars, and in the asteroid belt to increase life's chances of survival.

Advance science. Basic science provides the building blocks for breakthroughs that lead to better lives for humans and help us understand and protect our environment.

Encourage the market to develop space resources. Government/industry partnerships should lead to market driven, private enterprises. Once in space, private firms will allocate resources more effectively to develop resources for mankind's benefit. Only market forces can keep us in space for the long-term.

Some wery interesting discussion from perspective of the alt.space rocket builders:
http://lists.erps.org/archives/erps-list/index.shtml