What do you do with your Arduinos?

One in a small case, tied to a RasPi, it's an I/O buffer for the pi and has temp etc sensors, some switches, and a few display LED strings; the Pi has a little webserver and shows that data and mediates what the blinkies do in relation to switches etc. Couple years of accreted code and knotted wires and needs to be redone with an actual design.

When the weather warms enough I can use my hands again; I'm going to use one as the controller for a large dehydrator that will need several fan circuits, a heating element, vent open/close actions, as well as timers and a couple types of temp and humidity sensors.

Do the two serving in 3d printers count, even if the printers aren't currently operational and its someone else's design?

I've given away at least a dozen, with kits including a small breadboard and some LED's and so on, to kids as a "here make lights blink" set. Dunno if it did any good but I'll count it as a use.

For me the Arduino is either for learning or prototyping, because of quick iteration time. For learning a new technique (say a new protocol or a new IC like a shift register) I'll do everything on proto board until I understand it. For prototyping I'll do the same and eventually switch to a custom PCB.

ECG machine, uBITX SDR, and random small projects like a tempature sensor.