A good chair. I know it's not technically a gadget but if you're spending half of your day on the computer you should spend on a good chair with proper lumbar support. Your back will thank you.
External hard drive big enough to hold more than the entire memory of the computer. Keep everything you find valuable at least, or better yet back up the entire computer on there and update it regularly. Leave it unplugged from the computer between updates.
In other words, an offline backup of everything on your main computer.
A mouse. Just any mouse.
There are so many trackpad warriors out there (primarily Apple users) that complain they're being handicapped but they don't just go for the easy solution.
I've even seen some idiots stubbornly trying to do CAD work with a trackpad, and struggling in the process.
Get a multi port USB charger. I have some from Anker that have 2 usb-c ports and 2 usb-a ports. Can charge everything from my laptop to all my gadgets.
Monitor arms and/or a standing desk. Monitor arms free up so much space on your desk and having a standing desk is just good for physical health especially if you work from home.
if you draw or photoshop, a wacom screen is a life changer. not an ipad pro or surface, a 20"+ wacom cintiq on a solid desk is still light years beyond anything else out there. also, if you edit video, a usb shuttle wheel with mapable hot buttons makes cutting much faster than click and drag. really good speakers are important. lastly, get the best chair (with a headrest!) you can afford.
If you are a pc enthusiast I'm going to say you need a sensor panel. I built one myself, 7" lcd display and driver board costs less then 30€ and the result is incredible. I always look at it for any sort of reason it has temperatures, FPS, cpu/gpu/ram usage, network bandwidth.. I always know what's going on!
Appropriate game controllers for what you play. Gamepads, arcade sticks, racing wheels and flight sticks all have their uses. Mouse and keyboard isn't always the best control scheme and it's annoying to see steam reviews where people complain about that.
Anything you interact with on a daily basis. Keyboard, mouse, desk, monitor... If you're going to be spending a couple hours a day at it why not treat yourself?