BoingBoing Gadgets points to the Thirsty Light, a moisture sensor that alerts you when your plant needs water. The more your plant needs water the faster it blinks.
It’s very similar in function to Botanicalls (Radar post) — though not quite as proactive. The Asterisk-based system would call you based on light and moisture sensors. The Thirsty Light seems much cheaper and easier to have at home.