> and of course let's not forget microsoft: \\system\path\to\file

Nitpick: in Win32 you do not have the "multiple backslashes are equivalent to
a single one" guarantee that POSIX requires for slashes in unix file systems.
And paths starting with double backslashes are from a completely different
namespace (UNC), with very different semantics (i.e. no arbitrary path length
limit).

Actually most Win32 API functions ignore additional slashes, but a few 
unexpectedly (and of course undocumented) fail. (Mostly depending on the DLL
they are implemented in and their age.)

Martin