This tiny USB dev board made by Digistump has a huge amount of potential despite its low pin count and limited RAM. The brains of the operation is the ATtiny85, a cheap, 8 pin, 8 bit microcontroller. What really makes this board interesting is the USB functionality. You can program it using the Arduino IDE and use the serial debug feature, read sensor values from the ADC, and send values (like the color of an RGB light). All over USB. Very Cool.
On the ATtiny85 you get four ADC pins and three hardware PWM pins. The ADC is 10 bit, which is kind of surprising for such a small and cheap chip! In total there are six I/O pins available to you. If your program actively communicates via USB during operation (not including USB programming) then you lose two of these. The USB bootloader takes up 2K of RAM, leaving you with 6K. Still plenty to do some cool projects with. Some example project ideas mentioned by Digistump are pranking your friends by sneakily plugging this into their computer and having it randomly toggle the caps lock key. This is sure to drive anyone insane within a few minutes! Another idea is adding in an RGB LED and using it as a status indicator. For example it could notify you of a new email, or you could have the color represent CPU usage.