Back in the 1970s I made a multi-track tape recorder out of ordinary domestic tape recorders and a few extra bits and pieces. I didn't invent multi-track recorders of course, but I'm the only person I know of who made one that way, and it was obviously a lot cheaper than buying a studio recorder. The sound quality was a bit limited but it worked. I recorded tons of music on it, playing all the instruments myself.
About a year or so ago I made a water bath for making dough rise. The wholemeal flour I use for baking bread didn't rise very well even in the warmest reasonable place in the house, so I set up this bath using a plastic box, a small plug-in water heater, a fishtank water pump (for circulating the water) and a horticultural probe thermostat. The dough rises as much as I need it to within an hour if I have the bath set to 44 Centigrade. Before that I was just using a container of warm water, but that was tedious because it cooled down over time and I had to keep adding hot water to keep it warm enough. Thermocirculator water baths aren't my own invention, most laboratories have them, though they're expensive. They make breadmaking "machines" but I suspect the affordable ones don't give as much control as mine does.
I found downloading music off the Web tedious because on the websites I use, I have to do several clicks to download one track. So I wrote a macro to download entire albums automatically. It works very well and I've got tons of music that I'd never have otherwise had the patience to download.
I wrote a program that compares the contents of 2 different drives and creates a list of all the files that are unique to one drive. I have a lot of files and I store them in duplicate on 2 archive drives (because if I only had one drive then I'd lose everything if the drive packed up), and most of the programs that copy files from one drive to another sometimes fail to copy everything without providing a log of which ones failed - Windows drag-and-drop is always doing that - and it's easy to get into a muddle and lose track of what's been duplicated and what hasn't. There are lots of duplicate-finding programs around, but mine is the only reliable uniques finder that I know of.
I created some music mp3s at a range of tempos from MIDI tunes. I use them in an mp3 player to self-regulate the speed at which I pedal when I'm using the exercise bike. It's a bit more fun than using a metronome. Using music to regulate the speed of marching isn't new, but I don't know of anybody using it for an exercise bike.