Here is simple PIC microcontroller based scope. Looks like the software has quite robust features.

“SISAM - SImple SAMpler - is a simple PIC-based hardware for precision dual channel data acquiring with moderate resolution (up to 500 ksps in continous mode and up to 5 Msps for very short sequence). It uses USART and SSP for sampling signals and software routines for RS232 communication. USART is configured for synchronous master reception, while SSP is set for synchronous slave reception and is driven by USART clock via external circuit (wire).”

Via: HG Forum