It's all too easy to forget to water your plants. There are all sorts of tricks out there to automate the process, but Instructables user randofo's Arduino-powered system might be one of the more ambitious.
The parts list for this project is long, but the end result is a system that keeps your plants watered and alive without you even thinking about it. It uses a toggle, water pump, battery power and a few relays. It's actually very easy to set up, and randofo's Instructable is clear and easy to follow. If electronics aren't your thing, you can also do a similar system with just a plastic bottle and a tray. Head over to Instructables for the full guide.
Self-Watering Plant [Instructables]