Arduino

I use a ton of different types. I think my favorite is the EM deflection sensor, which is normally advertised as a "microwave radar" (it's not really either) sensor:

https://www.aliexpress.us/item/2251832601397384.html

The output is binary so it's easy to wire up. The sensor works through non-metallic surfaces so I can put it inside work boxes and still detect movement nearby. It's really fun to make Halloween jump scares with it since it can be totally invisible to people.

I'm currently starting to use LiDAR sensors. It's more work to do the background subtraction, clustering, and interpretation, but it's coming together. I want to make skeleton heads on servos that track people coming up my walkway.

For now, I'm working with Slamtec's RPlidar A1. It's cheap, does 360 degrees, and has basic libraries available: https://www.slamtec.com/en/Lidar/A1/