In the Windows Cygwin environment, uname is included in the "shellsutils" (may be called sh-utils) package available from the package manager (in this case, installer and the package manager are the same thing). Uname itself is not available as a separate package.
Example
Darwin Takuya-Muratas-Computer.local. 6.8 Darwin Kernel Version 6.8: Wed Sep 10
15:20:55 PDT 2003; root:xnu/xnu-344.49.obj~2/RELEASE_PPC Power Macintosh powerpc
This is the result with the -a option set.man uname
UNAME(1) System General Commands Manual UNAME(1)
NAME
uname - Print operating system name
SYNOPSIS
uname [-amnpsrv]
DESCRIPTION
The uname utility writes symbols representing one or more system charac-
teristics to the standard output.
The following options are available:
-a Behave as though all of the options -mnrsv were specified.
-m print the machine hardware name.
-n print the nodename (the nodename may be a name that the system is
known by to a communications network).
-p print the generic processor type.
-s print the operating system name.
-r print the operating system release.
-v print the operating system version.
If no options are specified, uname prints the operating system name as if
the -s option had been specified.
SEE ALSO
hostname(1), machine(1), uname(3)
STANDARDS
The uname utility conforms to IEEE Std 1003.2-1992 (``POSIX.2'').
BSD January 26, 1994 BSD