The One Essential: The Grinder
Every coffee nerd will give you the same lecture: pre-ground coffee just doesn't cut it. The full flavor and aroma that make good coffee so intoxicating is mortally wounded when ground coffee is left to sit for long. Fancy packages, plastic pods, and inert gas flushing do very little to change this cruel fact.
The ubiquitous blade grinder works by chopping the beans, not unlike a blender, resulting in non-uniform grind size ("dust and boulders") that can lead to some over-extraction or bitterness when brewing. Most of my colleagues in the strident forefront of the coffee trade will tell you a meager blade grinder is a nonstarter, but the truth is even an imperfect fresh grind is better than buying pre-ground beans. Some brew methods are sufficiently forgiving that you can still get very nice results with cheap blade grinding. Blade grinders are easily found for well under $20 dollars and they all produce roughly similar results.
Not cheap, but far and away the best investment you can make in your coffee universe outside of buying top quality fresh roasted beans. A burr grinder crushes the beans between two sharp burrs—one stationary, one rotating—and adjusting the gap between the burrs lets you dial in a specific grind size and produce relatively uniform grinds for better extraction.
Many people make the mistake of purchasing expensive drip brewers with blue LEDs and timers and numerous buttons and then balk at dropping dough on the grinder. This is exactly backwards. A good burr grinder is a unmatchable precision tool whereas any automatic drip machine is merely a fancy way to dribble hot water on your grounds.