When I do assembly programming, I generally use (in Windows) MASM 6.15 (integrated in Visual Studio .NET) or MASM32 if I have to do Win32 programming (which I generally try to avoid) and (in Linux/UNIX), I use NASM. I also do quite a bit of MIPS programming (processor used in Palms, Playstation, SGI boxes, etc.), and for that I use either PCSpim 1.0 or XSPIM (in Windows and Linux/UNIX, respectively).

AJ