Software Defined Radio (SDR) is amazingly cheap to dip your toes into. For about $40 you can get a little kit of a receiver, and a few antenna that can listen t

Software Defined Radio (SDR) is amazingly cheap to dip your toes into. For about $40 you can get a little kit of a receiver, and a few antenna that can listen to almost all local transmissions, radio, aircraft, navigation, ham radio, etc. GNU Radio is the toolkit that can let you do almost anything with it, using a GUI front end. With FreeCAD and a 3d printer, you can make almost anything, including gears, which I'm especially interested in. With the Arduino IDE, you can program a huge swath of controllers to build almost any control system you need for projects using the above tools. Gage Blocks are magical, you can spend $200ish, and have precision measured in microns. All of the above can be leveraged to do almost anything thanks to youtube, and all the people sharing their hobbies. If you want to measure micrograms cheap, or make almost atomically flat surfaces, etc... there's likely more than a few tutorials on the subject. Thanks to the internet, you could run your own forum, or use an existing one, to meet up with others interested in any given subject.