A thermostat. Just a thermostat.

From the Nest blog:

Complexity is easy. It’s fun to come up with new ideas and it feels great to say yes to everything, to avoid making trade-offs.

It’s simplicity that’s hard. To make a great product, you have to define its core – a single challenge – and painfully, painstakingly eliminate creative features to stay true to the product and its purpose.

Via Jason Fried.