Small, fast, cheap

Anyone looking for a cheap drahltlose connection for multiple MCUs is the nRF24L01 moduli quite good about it. Driver (MIRF, NRF24) there are in the network some of unterscheidliche applications, also for connection to a Raspi. However, if you want to test more than a little, can not get around the Nordic sheet and must be a bit more intense to deal with the details.

One drawback: the modules are controlled via SPI and need so a minimum of 4 data lines. Small solutions to the ATtiny85 are so very limited. But because of the low electricity consumption, the bi-directional data connection (half duplex) and the competitive price, the modules are then be interesting for some applications. The range is highly dependent on the environment, 5 to 20m in the house.