The official arduinos are very nice

In some cases though you may need something smaller or different like the boards mentioned by
tedderThere is also the dorkboard
http://www.dorkbotpdx.org/ which sells for about $10
It is incredibly small, it is half the size of a RBBB from modern device, 1/4 the size of a BBB from modern device, about 1/4 the size of an iDuino from Fundamental logic and about 1/6 to 1/8 the size of the official Arduino
Unfortunately the Dorkboard uses 2 SMT resistors which can be very difficult to solder, has a 5pin programming header which does not work with the 6pin FTDI cable, as far as I know, and does not have a voltage regulator onboard, which is a good thing and a bad thing
You either have to power it with a battery or an external regulated power supply
I have several of most of the boards
tedder mentioned and a dorkboard and a few other boards
