I'm getting quite keen on the Arduino devices. While they are nowhere near as powerful or easy to program as my beloved .NET Microframework platform they do come in a huge range of prices and configurations.
The Squareware 2.0 device you can see here is no exception. It is really designed for use as a wearable device. It has nice big holes that can be soldered into press-studs or connected to conductive cloth. It also has an on-board rechargable battery and recharging circuitry for an off-board one too. There are temperature and light sensors along with a buzzer and I reckon it is a pretty complete package. It also has a pair of quite beefy drivers so it can switch more current into devices like larger lights or motors.
You can pick it up from Tindie for 23 dollars, which I think is a reasonable price. Postage is reasonable too, and mine got here in only a week or so from the 'states.
I've not done anything with mine yet, but I'm sure I'll think of something...