There's always the chance for mining planets that are un-inhabitable. You can have mining colonies that require supply ships (to bring the esentials from a neighboring system/planet.) That could go over for the gas giants as well, some kind of star wars like gas mining operation. Also for neutron stars (with sufficient tech of course) to make exotic materials for even higher tech/armor....
There was a neat game called Stars! once that I played for a while. The graphics sucked, the systems sucked but the ship building and colonization/supply were awesome. Let me know if you want a link/copy.
Hope that helped some.
[edit to add]
What the heck:
http://www.download.com/Stars-demo/3...ml?tag=lst-0-1