Doors:

Livin-It-Up
savingthesims28_LIU5 - Secret Passage
More Livin-it-Up :yg Here for list of groups


Simlogical

Files saved with Saving Sims :yg Here.
Door that only allows NPCs to enter

http://www.jendea.net/
Archways (no frame)

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Windows (great for providing light in dark, windowless areas):

http://home.comcast.net/~lisimcreations/index.html
Under Objects

http://simcrafters.parsimonious.org/~magicsims/ found in Build / Pollys House Mouse Build
Invisible courtyard window

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Stairs & other Misc Items:

http://theblist.com/Drop/
Invisible Stairs - Blist site is gone but downloads will be back soon.

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*Thx to the following for contributing info.:* :thx
Susanne (Madoria)