Well... I might as well contribute.
One-way windows
(entrance in, but not out)
Used in The Last Day

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Shield Door Pool
(crouch for protection, great for infection games)
Used in Kinsythe

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Underground Tunnel System
(Might not be the best choice for competitive maps, but still really cool)
Used in Water Temple
Going in

Underground

Coming back out

Note: Because the window panel goes into the open box, it makes a small platform that can be used to jump out. Otherwise, it would not work or look the same.
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Umm... yeah, don't really have much else to post. My specialty is in infection maps so I thought I'd add some to the list, like these, that are helpful to gameplay. I also gave the links to the maps that used the structures for reference to their use.