This is a little thing I've cobbled together from memory and the Comixpedia comics wiki (which, by the way, is in desperate need of editing (which is why I relied on memory)). I had not seen such a graph of crossovers before, and I found this to be a terrible omission. To remedy that, here is the Reb Webcomic Crossover graph!

Absurd Notions     HOSERS
        |            |
General Protection Fault--Help
         |    |           Desk
Clan of  | Kevin & Kell
the Cats |
   |     | The Class Menagerie
   |     |  |*        |      |
   | Funny Farm---Newshounds |
   | |                       |
  CRfH!--- --Life at Bayside |
  ||      |                  |
 Fans!   Joe      The Suburban
 |  |  Average       Jungle
 |  |
 | Knights of the Dinner Table
 |
It’s Walky!--Melonpool--Zortic

Dork Tower   Beyond   Parallel
     |       Reality  Dementia
    PvP         |        |
              Emergency Exit
 The
Wotch--The Accidental Centaurs

Of course, I ignored cameos, one-ways, spinoffs, and anything else I may have forgotten (which made ignoring them easier). Here are explanations for some of the symbols, plus a running commentary.

|
Starting slow. This is just a vertical connector.
I started the graph by going from A (Absurd Notions) to Z (Zortic).

-
A hyphen, though it's not used like that in the graph (yet). In groups of two or more, it's a horizontal connector.
I've included links to those webcomics I read on a regular basis. (Dropped GPF a while after Eric Burns did, right before the main cast jumped to the evil twin universe.)

|*
This is an unorthodox crossover; while characters from "Funny Farm" appeared in "The Class Menagerie" at about the same time, in about the same place, as characters from the latter appeared in the former, this is much more like two one-way crossovers. (So is that more like half-duplex or full-duplex?) Good thing it's not on the main line!
The GPF/Funny Farm crossover was one of the stranger attempts IMO, since the main characters of the latter look like furries. (Actually, R. Smith has said that's only to differentiate the main characters. I wonder how that works with the "Newshounds" crossover. Ah well, I guess it's flexible.) I was fully expecting another comparison between Mr. Westone (FF) and Sean Connery (GPF).


This is the only instance I have seen of more than two comics (three, obviously) in the same crossover. Form follows function.
This is where I toss up my hands and say, "What's the deal?" Comixpedia has bios for nearly all the "CRfH!" characters, but nothing about the crossovers. It's not as if it hasn't reached a threshold; the "GPF" article lists five crossovers with five webcomics, and "CRfH!" has had that many, in a configuration that should be considered much more interesting but is instead ignored! By the way, the current "This Week!" article on the front page is no less than six months old! There's just no excuse for that! (pant pant wheeze)

||
These two vertical connectors simply represent two crossovers between the same two comics. This counts for two crossovers and the fifth webcomic for "CRfH!"
In my opinion, the most underappreciated (read: I enjoy them, but Eric Burns doesn't read them) comics on this graph are "Absurd Notions" and "The Wotch."


Empty space (I know, I know. I'm running out of symbols.) means there is no crossover.
I have made a little effort to alphabetize the titles in small areas, subverted by my efforts to keep the lines straight and the graph compact. Final size: 25 lines x 30 characters, about 600 total characters. The top connected portion's only 18 lines.

EDIT, 12-23-07: Nuts. The helpful Snap Shots are interfering with the three-way and the Class Menagerie/Suburban Jungle connectors. Rather than edit the graph (since the icon is like 2 2/3 characters wide), here's a link to the latest version as of this date. Alternatively, click the "comics" tag above.