The $10 Logic Analyser
The software is Sigrok PulseView, an open-source project that supports a very long list of measuring equipments.
The driver for the FX2LP CY7C68013A Mini Board is fx2lafw.
PulseView then lists 16 channels:
Basic measurements can be performed, like time and frequency, here SPI at 8 MHz.
However, the major limitation is the speed: with a 24 MHz crystal on the FX2LP CY7C68013A Mini Board, the maximum speed I could reach was 8 MHz, with frequent crashes starting at 12 MHz.
This is not fast enough for tracing a SPI bus running at 16 MHz.
Albeit its limitations, this affordable harware and software solution allows to explore logic analysers, use them on basic circuits and decode popular protocols like I²C and SPI.
For serious work, there is no secret: a professional logic analyser like the Saleae is a must.