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.
Essays can (and have) been written about what keyboards to buy.
It's very easy to spend $100 on a keyboard that isn't very good; moreover, you won't necessarily know, because most keyboards aren't very good. It's also very easy to spend $3-400 finding the right keyboard, because mechanical keyboards—and you should get a mechanical one, unless you go with Topre—are all subtly different from one another. That's why I now have four.
A keyboard lending library might be a good idea; unfortunately, it appears I'm the library.
If you don't want to spend the time on ...
What can I purchase with $100 that will be the best thing I can buy to make my life better?
I've decided to budget some regular money to improving my life each month. I'd like to start with low hanging fruit for obvious reasons - but when I sat down to think of improvements, I found myself thinking of the same old things I'd already been planning to do anyway... and I'd like out of that rut.
Constraints/more info:
Background:
This is a question I recently posed to my local Less Wrong group and we came up with a few good ideas, so I thought I'd share the discussion with the wider community and see what we can come up with. I'll add the list we came up with later on in the comments...
It'd be great to have a repository of low-hanging fruit for things that can be solved with (relatively affordable) amounts of money. I'd personally like to go through the list - look at candidates that sound like they'd be really useful to me and then make a prioritised list of what to work on first.