I already wrote posts about two great microcomputers - the Beagle Born Black (BBB) and the Raspberry Pi (RasPi). While the BBB might be superior for hardware projects, the RasPi represents a great multimedia microcomputer with an universal field of applications. But if you are really into custom hardware, the Arduino Uno (Uno) might be the right choice.
In the end the best combination is probably given by a RasPi powering / driving a Uno board. This combines the advantages of a full ARM computer with the customization, flexibility and directness of a microcontroller. You really get what you see. Additionally you are back to a world of 16 MHz clock speed and 32 kB of memory (for everything - data + instructions).
At first this might sound like a problem (and sometimes the memory limitation will not be the most helpful restriction), but it is certainly really useful in communicating with devices or manipulating other analog hardware. In general the RasPi is superior for everything digital, however, if you search for a good hardware gadget that might be useful in digital to analog conversion, the Uno is probably the one you looking for.
Right now I did not do too much with the Uno. I played around with the soldered LEDs on the board and I connected it with a small pinboard. One thing to watch out for is the voltage difference between the RasPi and the Uno. The Uno is relying on 5V wheras the logic of the RasPi is build upon 3.3V. Directly connecting the two will probably damage the processor of the RasPi. Connecting both should work great using the I2C connection. Again, don't forget to use a Logic-Level-Converter for using the correct voltages.
In the end I can only recommend Daniel Wilson's video on YouTube. He made some nice explanations and shows everything that is needed for setting up the Uno to work with the RasPi. Its important to realize that software is not the issue here, the video is focusing on the hardware. Once the connection is established, the fun can definitely start!