I recently went to an event where Arduino microcontrollers were mentioned, so I thought I'd try them out.
Arduino is an open source hardware and software platform, built around the Atmel AVR family of microcontrollers. These are very tiny computers by today's standards. They're only 8 bits wide, instead of the 64 bits of most new Intel servers and workstations. They only run at about 16 kilohertz instead of 1+ gigahertz. They only have about 32 kilobytes of permanent storage, instead of the multi-gigabyte disk drives on the machine you're probably reading this on. And finally, they contain only 2 kilobytes of RAM, approximate 1 millionth of the RAM standard in any laptop sold today.
But they're cheap, tiny, run well on a few AA batteries, and are well supported by the "maker" community. This is a large group of people who build their own projects, often hardware and software. Make magazine is devoted to this community.
What am I going to do with my Arduino? I'm not so sure yet. I bought enough sensors to build a simple alarm system, and I'm interesting in controlling my Arduino with a standard infra-red remote control. I might also use it to build a wireless mesh network. This project is mostly an excuse to solder some circuits and get by to my electrical engineering roots. I'll create small blog entries here if I do anything interesting.
I bought my Arduino, sensors, motors and a few tools from Adafruit Industries. There are literally hundreds of sites that sell and support Arduino and AVR. I like Adafruit because the woman that runs it has many great tutorials and helpful blog posts.