Can I land a plane in a field? If I had a plane that could take off and land in a short distance (eg., the SD-1 Minisport), do I have to use a recognized airport in order to land in the US?  Or can I just land anywhere so long as it's class G airspace?

FYI, this question is for non-emergencies.  IE., if I just wanted to land there because it was convenient.
Landowner's permission is always required.  Then check the municipal zoning regulations and the like - city, state regs etc.  Then your insurance company.  If all these points come up green, go do it.