My interest in hardware was basically sparked by the "9 dollar computer". It is a Single-Board Computer (SBC, the same category that the Raspberry Pi belongs to) that was actually announced before the Raspberry Pi Zero. It was a very interesting machine while it lasted - and in some facets, there still isn't a good replacement - but ultimately the company that made it didn't survive. The community was already at the point of collapse at that point, and people were forced to move on to other boards
I have a number of other similar boards as well. I use an Orange Pi as the server which hosts my git repositories as well as the project management software I use in my spare time. A Raspberry Pi is functioning as my media center by running Kodi. The Pinebook and Pinebook Pro are laptops built around an SBC. A number of other boards are in my possession, but not actually fullfilling any purpose right now.
My main focus has shifted to microcontrollers, though. This is the category which Arduinos belong to. As a quick summary of the difference between the two types of devices: an SBC is actually a complete computer that runs an operating system and can easily do many things at the same time, while a microcontroller is a chip that runs nothing but your program. Both have their pros and cons.
The limitations imposed by the hardware of microcontrollers both inspire creativity and create the challenge of overcoming them. The simplicity of running a single program on them and the ability to affect the real world (even if just by blinking an LED) make them a great teaching tool in my opinion. Ultimately, though, I think that the amazing capabilities of recent boards which can be bought for next to nothing is what really drove me to the microcontroller side of life.
At the moment, I still use the Arduino IDE to work with my microcontrollers. I think that it's a great tool to get introduced to the world of these devices, but ultimately, more complex projects run into its limitations pretty quickly. I don't really see the point of programming in another language, so I would like to continue working in C++. I have heard that there are hidden drawbacks to using the Arduino libraries, but I'm not quite sure yet if those are something I am willing to part with. The IDE itself, though, is far too limited and something I'm definitely looking to replace.
My favorite micro-controller is the ESP8266. The ESP-12 is the package of the microcontroller I use the most. The D1 Mini (designed by Wemos) is the development board with a ESP-12 on it that I often use. Being able to power and program the board over micro-usb is a major benefit and being able to solder on headers for the GPIO (and less GP pins) helps too. The D1 mini does this in a pretty minimalist fashion, which also keeps its price low.
The newer ESP32 (which is a follow-up to the ESP8266, the numbered naming scheme isn't exactly the best) is a device that I will use when I need the capabilities it adds. However, usually the older model is actually enough, and it's basically always cheaper. I own a couple, but I actually haven't done anything too serious with them.
Perhaps the most interesting thing about the ESP is that because it has wifi support out of the box, it can be useful without anything plugged in. The same advantage - combined with their amazing price - is also what has seen them getting included in many other product, such as smart light bulbs. Reprogramming an existing smart device using an ESP is definitely something that I'm planning to do some time.
Something that I'm currently looking into is designing my own boards. This would mean desiging the PCB, having it made cheaply in China and finally doing SMD soldering to get the components on it. This is in a very early "curiosity" stage and I might still end up dropping it all together. Still, I do think it looks quite interesting, which is why I wanted to mention it.