Free X11 server options

cygwin/xfree86 is a decent, free X server implementation.  I imagine
there'll be a cygwin/x.org implementation as well, eventually, if there
isn't one already.

I know you can forward auth to AIX from cygwin/xfree86 using the version
of openssh that's part of cygwin.

You may also be able to forward xauth credentials using putty.

xauth is much better than xhost, especially xhost +.

Another alternative is VNC.  VNC is free, stable, and fast on low
bandwidth links.

RealVNC's new protocol appears to be even faster than most other VNC
implememtations (there are many forks).  However, TightVNC appears to
be easier to build than RealVNC on AIX - also TightVNC implements more
of the modern X extensions that some applications need.

VNC is not a secure protocol out of the box - the initial password
is reasonably well secured, but everything else is open to sniffing,
so if you ssh somewhere and su from within your VNC session, that's
subject to replay attacks.  It's a good idea to tunnel VNC over ssh,
zebedee, openvpn, pptp, ipsec, or whatever.  Zebedee claims to give
better performance over low bandwidth links when tunneling VNC, than
native VNC does.

As an added bonus, with VNC, I've found that I can be semi-productive
from a wireless palm pilot by using PalmVNC and interacting with a RealVNC
server on RHEL 3, from there ssh'ing into my AIX boxes and other systems.
It needs a "special unix keys" popup though - it's hard to type "|", "~"
and such.  I'm in communication with one of the many PalmVNC developers
about this.  Still, one can easily use most GUI applications, and one
can check on the progress of long running jobs.

I have info about tunneling VNC over a couple of protocols at https://stromberg.dnsalias.org/~strombrg/vnc.html

I have more info about tunneling in general at:
https://stromberg.dnsalias.org/~strombrg/Poptop-notes.html
https://stromberg.dnsalias.org/~strombrg/VPN-info.html

Another possibility is to use Linux or other free operating system that
comes with the X.org or XFree86 X server.  You can do this by:

1) Using a computer that runs linux, or converting a computer to run linux

2) Using a computer that dual boots linux and some other operating system
- so you reboot into linux as needed for X11

3) Leaving whatever OSes a computer already has on it, and running a
"linux live CD", which means that linux is stored only on a CDROM,
and never needs to touch your system's hard disk

4) Using a system that can install linux into an existing windows
partition, like Lindows.  This way, you end up with a large file inside of
one of your windows partitions, and a linux filesystem is stored inside
of that file.  This way, you don't have to shrink any filesystems and
repartition, as you likely would with option 2 above.

You can also use this CD-ROM:
http://xlivecd.indiana.edu/

A newcomer on the scene is Xming, which you can find at http://wiki.freedesktop.org/wiki/Xming

It is a native port of xorg (?) to windows using the MinGW free compiler.
It is not dependent on Cygwin, unlike Cygwin/XFree86.  It also does
not include all the X libraries and clients - it's just an X server -
but it -might- have something that facilitates tunneling X for security
via part of putty.