There's two types of the cheap small arduino clones about. There's the Nano and the Pro Mini.
I've got some of both but I much prefer the Nano type. They come with USB serial built in and possibly more pinouts, slightly larger and marginally more expensive I think. You can power over the USB connection too which is really useful because I've got a load of mini-USB phone chargers from old phones lying around. I believe you'll need a Nano for the nRF24 adapters because the Pro Minis I have don't have a 3.3V pin which is required.
If I've still got a spare Nano I might make another wireless temperature sensor today for a different room.
EDITED: 4 Jan 2015 11:45 by ANT_THOMAS