Raspberry Pi is small enough to do many things, but if you still need a smaller option, the following coin-sized Linux computer VoCore may be more suitable for you.
The VoCore is an ultra-compact Linux computer with WiFi that measures 25 x 25mm. As shown in the images, the Linux computer sports a coin-sized body, so you can use it for many portable purposes. The ultra mini computer is powered by a RT5350 (360MHz MIPS) chipset and comes equipped with 32GB SDRAM, 8GB SPI Flash memory, built-in WiFi connectivity, and many interfaces such as 10/100M Ethernet, USB, UART, I2C, I2S, PCM, JTAG and over 20 GPIOs, so you can use the Linux computer to wireless a USB device such as printer, scanner, hard disk, camera, and etc, or use it as remote control robot with camera, portable VPN router, wireless speaker, offline downloader or remote control for Arduino, of course, more possibilities are up to your creativity.
The team of VoCore is raising fund via Indiegogo. Pledging $40 will let you own the coin-sized Linux computer with a dock. If you like it, jump to Indiegogo official site for full introduction.