OK, listen up kids, 'cause I'm only going to say this once. I think.

DIX: Driver-Indepdent X. Anything that isn't server-specific (extensions, core functions/structures, et al), goes in here.
DDX: Device-Dependent X; the inverse of the above.
XFree86: They still exist. They just released 4.4. The XFree86 distribution contains a DIX, a DDX, X libraries, X apps, fonts, and docs.
X.Org: They have a tree forked from XFree86 that contains all the same stuff, and still uses imake. They're working on a release. They're open and stuff now.
xserver: The freedesktop.org xserver project has a DIX and three DDXes - KDrive, Xizzle, and XWin. That's it.
KDrive: 'Keith's Driver', formerly known as TinyX. A completely separate DDX to XFree86 - very small, used as a testbed for stuff like RandR, Composite, Damage and Fixes. Good for embedded machines.
Xizzle: A fork of the XFree86 DDX, built with autotools, et al. Only just starting to link an actual binary, doesn't work yet, but is moving very, very quickly. Also, the binary is half the size of XFree86's. Pronounced 'shizzle'; mea culpa.
XWin: Cygwin's server for Windows.
freedesktop.org: We have xserver for the server, xlibs for the libs, and xapps for the applications. Everything's modular: the release schedule of the ATI driver is no longer tied to that of the X wire protocol, or some random fonts. Word.