We have fallen in love with these things here at Nomadic Research Labs, and are using 15 of them on the research vessel Nomadness to take care of all embedded control and data collection (see this blog post for overview and drawings). The Arduino boards are cheap, low-power, and exceedingly easy to use: just plug them into your Mac/Linux/Windows machine with a USB cable, run the development tools freely downloadable over here, and start playing. The language is a rich dialect of C, with such niceties as direct pin number references and painless support for I/O and serial communications, along with a very quick development cycle. The community is highly supportive... have a browse around the resources and forums at the Arduino site and you'll likely be drawn in. Since discovering these, I have abandoned all the other development boards that have accumulated here in the lab.
The Arduino Duemilanove (Italian for "2009") offered here is a microcontroller board based on the ATmega168 (PDF datasheet). It has 14 digital input/output pins (of which 6 can be used as PWM outputs), 6 analog inputs, a 16 MHz crystal oscillator, a USB connection, a power jack, an ICSP header, and a reset button. It contains everything needed to support the microcontroller; simply connect it to a computer with a USB cable or power it with a AC-to-DC adapter or battery to get started.
There are lots of shields available which plug directly into the board and provide workspace or specialized functionality (including ethernet, XBee wireless, displays, motor control, and general prototyping). We'll be adding to our inventory of these and related items as the product line develops.
