Adafruit BitScope Micro USB
Poke and prod your electronics with the BitScope Micro Oscilloscope and Analyzer! This micro USB powered oscilloscope by BitScope is tiny, light weight, and water resistant. It's also fully user programmable and comes with a set of ten mixed signal mini-grabber test probes and a USB cable for power. The BitScope works with a full line of BitScope software for Windows, Mac OSX, Linux, and Raspberry Pi.
The best part about all this is the functionality the mini oscilloscope offers while still fitting in the palm of your hand. You could slide the BitScope in your pocket and debug on the go or in the field with all the input/outs you need.
There's also an eight channel logic analyzer, arbitrary waveform and clock generator, and a spectrum analyzer - so it really has all you need to get started in understanding circuits in details, in a compact package.