AtomBusMon - is an open-source In-Circuit Emulator for the 6502, 65C02, Z80, 6809 and 6809E 8-bit processors.
It is built using an off-the-shelf programmable hardware device called a GODIL that can be purchased for just over $50. The device plugs into your CPU socket, replacing the CPU, and you can control it over a serial connection to debug your software or hardware.
You unplug the 6502 processor (which is in a 40 pin DIP socket) and plug in the GODIL loaded with ICE-T65 design. ICE-T65 contains a fully functional 6502 processor, plus a second arduino compatible processor running an interactive debugger application via an independent serial port.
Now you have a set of powerful debugging tools at your disposal: