Here’s a USB oscilloscope project from a few years back. It’s easy to build on a single-sided PCB and very cheap because it uses just a handful of parts. At the center, an ATtiny45 microcontroller uses its ADC capabilities for the two traces and also handles the USB connectivity. The internal oscillator is used and trimmed up for accuracy by referencing the USB clock. On the PC side of things, a program written in C# displays the data coming over the serial bus. Quick, small, and useful; a schematic, board layout, firmware, and PC software sources are all available for download.