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.

 

[zombiesniper]

Here's another one.
500mm butterfly by Trevor Baldwin, on Flickr

Used my 7DmkII with a 70-200 F4 at 200mm.

Shot through the 500mm at an object to frame and slightly magnify it.

500mm macro by Trevor Baldwin, on Flickr

 

[Jeff15]

Some very interesting shots here ...

 

[petrochemist]

I'll add another toy - a c-mount zoom microscope objective home mounted to a camera (without the original relay lens & mount).
My mounting allows infinity focus (with some vignetting)
infinity focus test - shipping by Mike Kanssen, on Flickr

Here's the lens & jury rigged mount
ultra macro lens set up by Mike Kanssen, on Flickr

Full magnification has 6mm covering the width of MFT so about 3x life size
full magnification in mm (unedited) by Mike Kanssen, on Flickr

 

[zombiesniper]

Very cool.