"...in software" should have been appended to the title.

Back in the day, I was quite heavy into the x0xb0x. (https://www.ladyada.net/make/x0xb0x/) It's an open hardware 303 clone by Ladyada and was designed to use as many of the original 303 components as possible. According to those who own both, the sounds are essentially identical. (But the x0x is much easier to use.)

Somewhere in 2006, I was too late the to party to snag one of the original kits, but a little cottage industry formed on the x0xb0x forums to support the community of people who wanted to build and mod their own. Adafruit provided the PCBs, the common components came from DigiKey and Mouser, the rarer components from eBay or other forum members.

I ended up buying enough components to build six, but only ended up building three. The first one I kept, the other two I sold. I recouped my cost with those so I also ended up selling the rest of the components later as my interest in building them waned. I should have held onto those and built the rest with my kids when they got older, since even the replacement components are hard to come by these days and they are still a fun project to build.