Outposts have been discussed enough under various other subjects, that i think we are pretty close to having the plan down. So rather than try to start from scratch, i'll lay it out as i see it, and we'll see if anybody says, "Yep, that's exactly what i'm thinking."
Outpost are sorta like colonies-- but different. They can be used to extend your empire in certain circumstances when a colony doesn't make sense. I believe outposts should be distinct from colonies. Conceptually they should be more distinct than the, "little colonies" that they are currently in FreeOrion. That implementation is due the current limitations of the engine.
* Can create supply lines (both kinds)
* Provide vision & detection
* Can be defended and invaded like colonies
* Have no population
* Produce no resources
* Are significantly cheaper
* Can sometimes be built where you can't put a colony (depending on your techs)
* Can be replaced with a colony (if you would be able to colonize without the outpost)
* Can be upgraded with outpost specific buildings
* Can be built in systems where there are no planets?
So: discuss, agree, disagree, propose, etc.
Here's an earlier thread on outposts.