Beneficii has went to NESDEV a few days ago and introduced a new version of FCEUXD SP called FCEUXD ABS and CDLDIS. This will allow you to create a copy of an NES game's source code. With some modifications, You will have a Complete Disassembly (Using the Code/Data logger from FCEU ABS and CDLDIS) of your favorite NES game!

The ASM language Beneficii is basing it off is a Assembler called ASM6 (a X816/ca65 Hybrid/Variant) by the famous Loopy of LoopyNES fame (He is still alive today in NESDEV)

Links:

http://beneficii.net/smb2/

http://home.comcast.net/~olimar/NES/asm6.zip

http://nesdev.parodius.com/bbs

http://nesdev.parodius.com/

Have fun,

(These utilities I mentioned are by Loopy and Beneficii and not by me. you can use them freely!)