"Retromaster" has published the A2601 which is "an FPGA-based clone of the legendary ATARI 2600 video console, including VHDL code and a custom PCB"

I should synth it and report the results. The code looks nice to me.

The other two VHDL cores I'd like to compare it with are T65 (the best version is from pacedev) and Syntiac's cpu65xx_en from FPGA-64

Edit: nichtsnutz points out that the cpu65xx_en is only for educational use - not open source.