For me, it a chess clock. I’ve got all the parts (esp32, oled screens, usb c charging electronics), the software is written, the case is 3d printed. All of this piled in a box on my desk mocking me. What about you?
Not a single project, but a graveyard of expired domains. I constantly hunt for good dropped domains to build niche sites and blogs on them. I end up buying them, feeling a rush of productivity, and then leaving them parked while I go back to maintaining my existing apps.
All of them. Drives of them. I'm too stupid and obstinate to quit. Sites from when I wanted to do web dev, a Hacker News clone written in Hack. Games games games, most of which are prototypes. A personal website and blog that's still only half done. Scripts and tools for the blog that are never done. Started teaching myself tkinter so I can make a UI for posting in my blog, so I can make a blog post about that, probably never get done. Short stories and cringe poetry yellowing in binders. I'm the walking embodiment of the sunk cost fallacy.
Nowadays I just put all my projects, apps, and games on subdomains under a single main domain.