The main thing you need to do to keep your plants alive is to water them. So, Instructables user dr.mcc built a moisture sensor that alerts you when you need to water a plant by using an emoticon.
Using an Arduino, a moisture sensor, an LED matrix and a few other parts, dr.mcc's build monitors the moisture level in a pot of soil and then lets you know when the plant needs water by displaying a sad face on the LED matrix. Reminding yourself to water your plants is never that enjoyable, but this way adds that extra special little bit of guilt to help you really remember.
More Human Moisture sensor [Instructables]