PiTubeDirect is a low cost Second Processor project for Acorn's 8-bit machines (Beeb, Master, Electron, Atom) which uses two cheap chips to interface a Raspberry Pi to the Tube connector.
The Pi emulates one of a number of CPUs, and also the Tube interface chip.
A Pi Zero can emulate a 6502 Second Processor running at 274MHz.
For more information, see the project Wiki.
Here's the minimal configuration, using a Pi Zero and a DIY two-chip level shifter: