Other than selling them, are there any cool projects that can be done with an old graphics card?

Hopefully, I'll soon be able to upgrade from my 980 (04G-P4-2983-KR) to a 3080. I was thinking of maybe doing an HTPC build with the 980.

I also remember reading an article where someone used an old graphics card to handle physx exclusively. It actually resulted in worse performance :P But I think stuff like that is really interesting.

I do a lot of hand me downs.  My prior gaming rig, has a 970 and pushes the Rift CV1.  My grand kids come over and enjoy playing the virtual roller coaster and such.
 
Once I get my new G2 and 3080, I'll push my 1080SC and Rift S over to the hand me down rig.  Then I'll need to figure out what to do with the CV1 and 970 GPU....

Like javelina1, I pass my rigs or cards down to my kids. I used to add my old cards to my FAH folding farm. 

Donate them!

Sell them on ebay.

I would keep an old card around to troubleshoot if my video card dies. Once it happened to me and I had to borrow a video card from a friend to swap in and verify that was the problem before ordering a new card. Would have saved me a week if I had a spare.

I run them in SLI and see if it crashes the system or if it will even post. You should try it if you have more than one.

They upgrade the others family PC's

I used to be very generous with all of my extra/old tech, until the day I actually needed like four VGA to DVI adapters (go figure). So now, I usually keep all of my old components and turn them into little servers for various tasks - you never know when you're going to need to use another computer to debug driver level code.

I’ve got 4 spare fans ile trade ya