The makers of the pcDuino line of products offer several small, low-power single-board computers with ARM processors and support for Arduino Shields.
The latest model is the smallest (and cheapest) to date. The pcDuino3 nano is now available for $39.
This tiny computer features an Allwinner A20 dual-core ARM Cortex-A7 processor with support for Android and Linux.
It features ARM Mali-400 graphics, 1GB of RAM, 4GB of embedded flash storage, a microSD card reader, HDMI output, 2 USB host ports, 1 USB OTG port, a 3.5mm audio jack, and Gigabit Ethernet. There’s also an IR receiver in case you want to use a wireless remote control, and a SATA host socket.
The little computer measures 3.6″ x 2.1″.
For developers, there are also GPIO, UART, and other pins and the same Arduino sockets you’d find with an Arduino UNO. This lets you connect existing Arduino Shields to add Bluetooth, a touchscreen, or more for testing or hardware development purposes.