My 2nd monitor. It's some 24" curved 165Hz 1080p monitor that I bought from a guy at my university for $105. While not the best for gaming (noticable ghosting), it's been incredibly helpful for work as it gets more complicated in university. The extra screen real estate lets me fit so much more without needing to alt tab or click on another window.
When I get a bit more space to myself, I'm thinking of somehow wall mounting a 42"-50" 4K TV and using that as a work monitor. Or maybe I don't need a wall mount. I'd have what feels like acres of screen real estate.