It was no small dribble. It was a veritable deluge. (Click the video below to see the deluge.) That may be a bit of an exaggeration, but there was water coming into the house and unsteady power. I tried patching the leak with hydraulic cement, but it couldn't stop the flow because as the water stopped, it pooled up outside and built up pressure. (Instructable for basement repair pending?)
With a partially blocked flow running into the sump, I needed to know if the power died in the middle of the night because if the sump pump stopped, I'd need to empty it with buckets. The pipe into the sump was dumping a lot of water in addition to the leak. No fun, but better than flooding. Carefully directing the little river through the basement to the sump and jamming a bit more hydraulic cement in the crack, I went upstairs.
With the flow slowly breaking the patch I made, I quickly programmed an Arduino to beep when the power went out. It took like 10 minutes because I had to call my friend for the analogRead values. It's 0 to 1023 for voltages 0 to 5V. I know, I know, I could have found it in the examples, but I was without internet and I was desperate and not thinking clearly. I should probably get outside sometimes …
The Arduino never actually woke me in the middle of the night. The power stayed, but it was a lot of peace of mind.
And then the next day I found that the internet box where the fiber-optic comes in was plugged into a tripped GFCI. Oh, the embarrassment. I promptly got on the internet and made an instructable after checking my webcomics.
I didn't end up running wires down two flights of stairs to trigger an alarm when hypothetical flood water shorts some wires. I wanted to, but I just let the sump pump deal with it because it was late.
(I'm entering this in the Arduino contest because, wow, this was the most useful thing I did with it. Vote for me or whomever you think is cool. So me?)
(Too bad the battery-powered contest just ended. This would be pretty good for that, too.)