Not for it's intended use.

[zombiesniper]

In this one the aim is to use your camera/lens setup to shoot something that isn't what most people would think that camera/lens is used for.

Add in the camera and lens make, model.

I'll start.

Sunset.

Canon 7Dkm2 with a Canon 500mm F4L
Settings 1/125 F4 ISO 160

Fire in the sky by Trevor Baldwin, on Flickr

 

[Space Face]

Good idea. I'll have a crack at this later.

 

[smithdan]

..just noticed this theme,

Poor guy's macro..
Slide projector lens on half a cardboard paper towel roll, handheld down front of a reluctant Pentax K S2

 

[zombiesniper]

Here's another one.

Landscape

Canon 7Dkm2 with a Canon 500mm F4L
Settings 1/500 F11 ISO 160

mountain by Trevor Baldwin, on Flickr

 

[petrochemist]

I'll give you a couple where the substance used for the filter is not in its intended role:
Using an old floppy disk (sorry about the finger tips)
IR via diskette by Mike Kanssen, on Flickr

ans using an old end from a negative strip:
IR via negative by Mike Kanssen, on Flickr

Both shot using a full spectrum converted Panasonic GF2 using an olympus 17mm/2.8 lens.

A more recent example uses a pencil case as a filter but I don't have that one on-line

 

[petrochemist]

This one probably qualifies too, using a telescope eyepiece as a 'close up filter':
Knot macro by Mike Kanssen, on Flickr

Shot using a 40mm eyepiece in front of a DA18-55 (at the telephoto end) mounted on a Pentax K100d, the image on the sensor being about 1.5x lifesize.

Maybe more in the spirit of the intended use of the thread I can offer a fisheye used for motorsports:
sml P1300143 by Mike Kanssen, on Flickr
Using an Olympus 9mm BCL on my Panasonic G5

 

[zombiesniper]

Those are great. Using other products to get an effect is perfect.