X Servers for Win32

Xming
http://freedesktop.org/wiki/Xming
Xming is the port of the X Window System (X.org) to Microsoft Windows.

Cygwin/X (formerly CygWin/XFree86)
http://www.cygwin.com/xfree/
http://freedesktop.org/wiki/CygwinX
Now supports rootless and multiwindow modes that allow the X windows to fit in nicely with your Windows desktop.

StarNet Communications Corporation's X-Win32
http://www.starnet.com/

Xappeal for MS-DOS
http://www.xtreme.it/pub/xappeal/
XAppeal is a proprietary X11 server for MS-DOS based on XFree86.

MicroImages, Inc.'s MI/X
http://www.microimages.com/mix/
Proprietary X11 server for MS-Windows.

LabF's WinaXe and WinaXe Plus (formerly X-WinPro)
http://www.labf.com/
Proprietary X11 server for MS-Windows.

NetSarang's Xmanager
http://www.netsarang.com/products/xmg_detail.html
Proprietary X11 server for MS-Windows.