Some high-quality computer peripherals: mouse, keyboard, or chair.

Many people spend a huge fraction of their day on the computer, so it's important to optimize that experience as much as possible. For a long time, I thought of that optimization purely in terms of hardware performance, or operating system, with maybe some time spent on files structure or monitor. But more recently I got a high-quality keyboard and mouse, and they've made a huge difference for much less than the cost of a hardware upgrade.

It's easy to forget about the material objects that you'll be in physical contact with, so they're low-hanging fruit for a lot of people.