A USB sound card can serve as a cheap, consumer off-the-shelf ADC, a technique pioneered by Marek Dolleiser. The data is sent to the computer as normal sound and stored in a WAV file. Specialized software processes the "sound" to perform pulse-height analysis and multichannel scaling, forming a complete MCA.
Sound cards have high-speed but low-resolution (up to 192 kHz) ADC chips, allowing for reasonable gamma spectroscopy performance for a low-to-medium count rate. The "sound card spectrometer" has been further refined in amateur and professional circles.