Daniel Caetano has released an Z80 disassembler for PC, which adds several comments about MSX hardware, making it most suitable for disassembling MSX code. You can find the tool on his website, which contains more interesting MSX software titles.
The disassembler runs on MS-DOS, OS/2 and Windows. It supports 4 different kinds of disassembling, each one for different needs. Furthermore it can identify BASIC binaries, MSX-DOS files and ROM images. Of course, suggestions and bug reports are always welcome.
Relevant link: Fudeba MSX homepage