Where can you store items you want to save?

I've seen a tip that says if you store items in someone else's container they may not be there later on. I assume that means if you put something in an empty box in an NPC's store they may take it, or if you put an item in the fridge of an abandon house the game may decide that some NPC has found it.

In Megaton I have seen that there is an empty house that requires a key to enter. Once you get the key, does that house become yours for sleeping and storage?

Thanks again for the great help everyone.

Yes to the Megaton question.
When you are out in the wastes, you will be able to store items in ammo crates, medical kits and nuka cola machines. These will never respawn, and will be safe all game. The various boxes and crates that are grey, WILL respawn, at least they do on 360, so storing items there are at your own peril.