Spirit Walker
An out of body experience
Spirit Walker lets you detach from your minecraft body.
Just drink the magic potion, and your soul will fly free, for a time, to explore your surroundings unhindered.
(it's freecam, with optional noclip)
But remember: your body will not join you on the journey.
Make sure to secure the area before drinking, or risk watching helplessly as you get eaten alive by zombies.
Recipes
Use warped fungus on awkward potion to get basic Spirit Walk, then further enchance with redstone to extend or glowstone to go through walls.
Splash and lingering not brewable.
Server owners and operators
Spirit Walker must be installed on both the client and server to work.
More importantly, clients that install Spirit Walker themselves will not be able to use it on your server unless you install it as well.