Arduino is an open source programming platform designed to give hobbyists a cheap, accessible and flexible way to add extra functionality to just about anything. The Arduino board supports all kinds of sensors, like light sensors or proximity movement sensors, and through programming, their readings can be used to take some sort of action. Example: Photographers can use an Arduino board to trigger a flash and picture snap based on sound. Here's a better example: A programmable planter can be configured to water a plant every time it measures the soil's moisture level dropping too low.
That's the key to homebrew gardening applications like the Garduino, which is truly a programmable planter. The Guardino uses sensors to detect moisture level and water plants. It uses a photocell light sensor to detect when the sun goes down, which triggers grow lights to provide a few more hours of nourishment. It even lights up a warning LED when the temperature drops too low. Garduino is a great example of how versatile homemade solutions are: It uses a pair of nails driven into the soil as conductors to determine moisture level. The sensor reports a lower resistance when the soil is especially moist.
All of the sensors plug into a simple Arduino circuit board, which costs about $30. Open source projects get posted across the Web and sometimes expanded upon by enterprising developers and tinkerers in the open source community. Garduino got such an upgrade with support for Twitter. The enhanced programmable planter sends out tweets containing sensor readings to reassure gardeners that everything's hunky-dory while they're at work.
Garduino is hardly the only Arduino-based gardening helper out there. Garduino and alternatives like a water pump-based system designed for tomatoes require a little bit of programming know-how and a willingness to cobble together lots of parts, but construction is part of the fun, anyway. Gardening is about building something, too, though not in quite the same way.