Dallas 1-wire
The Dallas 1-wire protocol is very simple to realize in hardware. It needs just 1 wire per sensor to communicate. It gets even better then up to 128 of these lines can be connected to one Arduino in/out pin. Of course this comes at a price. It takes a lot of time to start communication (5,4ms) and the max speed is 16.3 Kbps. That is not really fast. But for many applications it is fast enough. Especially famous are all the weather sensors by Maxim.

You can find a webpage on how to setup a robust micro network at Maxims. Once you have done that you can use Atmel to implement a 1-wire master. But for Arduino it is much easier to install a library. All sites I can find all go back to the Paul Stoffregen library. The example of this (DS1820) lib works fine for me. More info can be found at Arduino playground.