This simple USB oscilloscope allows continuous data recording at 2x300 kSps, 8 bit. Although it has very reduced analog part (low input impedance, no gain switch, only single 0-6.6V measurement range) it could be worlds third cheapest dual channel oscilloscope (right after sound card and this AVR circuit). It is based on STM32F103C8T6 - Cortex M3 microcontroller with Full-Speed USB costing around $4.
PCB dimensions: 66 x 36 mm, total cost 8 - 10 USD.
It works with miniscope v4 PC software:
Updated (2012.04.29) firmware makes use of both A/D converters allowing to increase input impedance (no crosstalk between channels). Sampling speed is increased to 2x461 kSps and S/H time is increased from 7.5 cycles to 13.5 cycles.
ADCs can still work faster, but this is most likely maximum speed that can be handled by USB full-speed and real time streaming is most important feature here allowing to use i.e. 2x128 k sample buffer with MCU that has much less RAM and store to file continuous stream up to 10^9 samples (limited by used file format). While this circuit can not by any means compete with bench oscilloscopes in sampling speed category it can beat even expensive ones in terms of sampling buffer size using up to 4 GB of disk space to capture waveform.