I know they are two different source trees but not much more than that. Where can I get a good summary of the differences between the two?

Thanks!

Peace...

i think the only real difference for end user is a "composite" extension in xorg is not in xfree yet
most likely because it is not stable with nvidia glx

I am sure this has been asked before so use the search function and look for similar threads.

I did search on "XFree86 4.5.0 vs X.org 6.8.2" didn't find what I was looking for. I guess I'll have to keep digging.

Peace...

well when it comes to software that is always changing the answer to a question like this changes constantly. And since he needs info for specific versions that may not have existed previously searching old threads about the differences that existed 3 months ago is pointless in the extreem.

I believe the more accurate answer is licensing.

From Wikipedia,

XFree86 4.4 was released in February 2004 with a change to the license [9]: the addition of an advertising clause , similar to that in the old version of the BSD license. Some hypothesised this was in response to the announcement of the Xouvert fork, to ensure that any fork of XFree86 would have to notably credit XFree86.

Many projects relying on X found the new license unacceptable [10] — the Free Software Foundation in particular as they held it to be incompatible with the GNU General Public License [11] (although XFree86 disagreed), and others simply did not wish for any code incorporated to be any further restricted than it already was. Some projects, such as OpenBSD, forked XFree86 from version 4.4 RC2, the last version under the old license. (OpenBSD later adopted the X.Org Server [12].)

The XOrg Foundation Open Source Public Implementation of X11

The X.Org Server is the official reference implementation of X11, produced by X.Org. The first version, X11R6.7.0, was a fork from XFree86 version 4.4 RC2, with X11R6.6 changes merged in. Version X11R6.8 added many new extensions, drivers and fixes. It is not encumbered by the XFree86 license changes. It is hosted by and works closely with freedesktop.org.

Most of the open-source Unix-like operating systems have adopted the X.Org Server in place of XFree86, and most of the XFree86 developers moved to X.Org [13].

Sighhhhhh

one way to try to track the changes if you are like me and don't give a rats &^% about the licensing
fake pseudo argument is to try to read the releasenotes.
one thing you notice is some of this stuff is copy paste from one to the other just as the code is largely copy paste. There can never, i would imagine, be a large divergence between these two projects because both intent to function in the same environment.
http://wiki.x.org/X11R6.8.2/doc/RELNOTES2.html
some of what is presented on this first one is clearly broken so one feature of xorg is that it is a bit unstable and the build is somewhat broken as per it's more aggressive stance.
As a matter of fact almost every release of xorg so far has been badly broken in some way.
The price of everyone having acess to the code. Xfree on the other hand is now and for the most part has always been stable.
xorg because it's traditional relationship with sun has always contained some sparc and java related fixes sometimes not in xfree yet for lack of direct access.
often features like xprint appear to be in xorg but not xfree but this is not the case or they may not include xprint in xfree i'm not sure. But the point is mute just like alot of it because xprint is maintained by others and can be installed on any x system. Often people building x will not build these types of included things because thay are always by definition behind the easily aquired otherwise.
http://www.xfree86.org/4.5.0/RELNOTES.html
xfree86 included freetype and fontconfig actually compile unlike xorg (not that you want to use them)
XFree86 will compile without a host.def file xorg will not despite documentation to the contrary
like i said above xfree does not yet contain some extensions like "composite" which in xorg is broken or at best a problem.