I’m a pretty good gardener and I can usually figure out when my houseplants need water, but for the green-thumbed-challenged among us, there is now the Digital Thirsty Light. You stick it in the plant’s pot and it blinks when the dirt has dried out.
Personally, I’m waiting for the Wi-Fi version. This one would be attached to a watering can and would notify me on my cell or PC that my plants are dry. Then I can send a signal from work and have the watering can tip over and give my houseplant a drink.