What debugger do you guys recommend? I'm using Windows now, but I plan on supporting both Windows and Linux with my program, and maybe Menuet too.

I haven't used a debugger since MS-DOS days, when I used Turbo Debugger. I also wrote a debugger myself for my 65c02 cross-compiler at about the same time. I don't normally use debuggers --- but maybe I will this time.

I know the HLA crowd uses Olly, but I haven't tried it yet. Am I right that FASM generates a debug file that can be used by any debugger?

I use Olly for all my work, including at my job and for FASM code too.
I find myself never using the "debug symbol" file. If I need to debug
some place, like a function -- I just insert INT3 at the beginning of it and
run the code in Olly - then step through code. I also have assertions in FASM
and some tracing code I wrote, like a dump of a structure -- all
using OutputDebugString, so it can be seen in Olly's log window.

As I heard in LINUX -- it is hard to find a good debugger there.
Olly is for Win32, I think.