Everyone should have a good chef's knife and know how to use it. Victorinox sells polymer handle stainless steel knives that are as good as some $200 knives I've owned for $50, probably other companies too, go to amazon. A single good Chef's knife is good for cutting almost everything you would need to cut on any cullinary adventure. Keep it clean and extremely sharp by using a sharpening steel, or, if you're not up to learning how, a knife sharpening tool, though those will slowly degrade the blade.
Buy the knife, learn the knife, it's amazingly easy to prepare your food with a single excellent knife rather than a slapchop or a bunch of specialized weird tools for cutting various things. There's a reason the professionals do it this way. Have a great time preparing your meals with much less difficulty, and thank me later.
Knives are tools. Tools that are excellent at a wide variety of tasks are rare. Yes, you can make do with a single chef's knife, but I don't see why would you want to. Even for basic kitchen activities, ignoring specialized tasks, I would want to have on hand at least three knives -- one big (e.g. a chef's knife), one medium (usually called a utility knife), and one small (a peeling knife).
Oh, and a steel is not used to sharpen knives, it's used to straighten out the knife's edge. To actually sharpen you will need a sharpening tool -- there is a large variety of those, differing in capability and the required skill.
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.