A Swedish programmer, engineer, and mechatronics expert, Linus Akesson, made an interesting device to detect the moisture level in a potted plant's soil. He stopped there, on the grounds that if it over-watered, it would ruin the wood floor under the pot. Based on his work, I think I have a completely automated plant care device.
His machine measures the soil's resistance to electrical flow. Moisture decreases this resistance. So, a microcontroller, connected to a probe wire and a relay, can probe periodically as in Mr. Akesson's design, but rather than merely recording and reporting this resistance, will, at a certain level, activate the relay, which will open a valve and water the plant. It can close it after a set period of time, or it can close it when the resistance has reached a certain level.
A microcontroller could also log this data to a computer, turn on or off an electric light for indoor or space growing operations, alert me if it runs out of water, or activate some sort of a camera and store the pictures so I get a "time lapse" of the plant's growth.
If I could only get it to monitor soil nutrition and chemistry too, then I've pretty much made a machine that automates plant care. And that's of interest to homeowners, gardeners, and farmers.
Mr. Akesson also does projects with musical microcontrollers, a chiptune piano, which he also demonstrates to great effect, and a rather good description of the historical applications of TTY technology and why modern computers support them despite very few people or institutions still having anything remotely similar to a teletype.