Ask HN: Most interesting tech you built for just yourself?

I suppose this counts: Although not that interesting, I have one of those AC units in my apartment that sits on the floor with an outlet tube terminating at a window to pipe out hot air. The internal reservoir for this unit is quite small so I hacked together a float valve that triggers a pump to offload the condensation water to a larger bin, that way I don't have to empty the reservoir as often.

Pretty simple, but saves lots of time and I don't have to worry about the air conditioner turning off in the middle of the night due to it's internal reservoir becoming full.