"Novices to the Way, I see," the little gnome squeaked. "Well, your worships have certainly come to the right place!" he added with enthusiasm. "Unlike some of our competitors, the Pavilion serves the main parallels -- and a few of the trunk lines, of course -- of the multiverse. We have no truck with the unhospitable planes, off byways, dead-end dimensions and the like. No, sirs!"
Taking them by the arms with his gnarled hands, the colorfully garbed gnome led Gord and Chert a few paces along the corridor. He gestured to a strange maze of shifting lines and glowing, pastel-colored dots displayed on the wall of an alcove. "There, see? All the routes that our gates serve are shown here. Fares are given in credits, domars, and sequins, as well as the standard precious metals, as displayed to right and left."
- Gary Gygax, "The Weird Occurrence in Odd Alley"And with such a brief, prosaic explanation are we given a description of the Way, a means of trans-dimensional travel that is well-enough established to have a regular industry supporting it. In this wonderful story, we are given glimpses of a multiverse that smashes beyond the mundane understanding of "the planes" as seen in the Players Handbook or DMG. These aren't your everyday gates to the outer planes; these are a fully-developed system of travel between alternate dimensions.
But what impresses me is the everyday business of it all. This series of gates is run like a railroad (complete with trunk lines), and is a sort of open secret amongst the well-to-do on the various worlds. It's even well-developed enough to have its own currency, and those In The Know are knowledgeable enough about the various alternate realities to know what a 1947 Margaux is. There are gates that connect to the system in the major cities of the Flanaess (at the very least Greyhawk and Rel Mord; one must assume others exist as well in places such as Rauxes, Niole Dra, and Irongate). Trade is conducted via these routes; the foppish Lord Maheal from the same story quoted above is in one of the extra-dimensional "sidings" of the Way on a mission to purchase wine to bring home to Nyrond.
The connections between these locations and the Way are carefully disguised and protected by magical keys; possession of a key presumes knowledge of where a suitable gate for its use may lie. Different keys-- some, at least, the size and shape of a common coin-- lead to different places. Or, maybe they merely open different gates which are linked to different places. We don't know for sure, and it may well be a combination of the two; use key A in gate B, and it takes you to plane C. Use it in gate D, and you end up in demi-plane E.
Not all of the destinations are full-fledged worlds. Sure, some gates will take you to a tower in the king's castle in Rel Mord, or to a seeming dead-end in the seedier end of Greyhawk. But others will take you to what I might call a "siding"; places under a mile in diameter, whose verges are clad in gray mist, such as Odd Alley, where one can find a brief respite on a long journey, facilities for transferring from one line to another, and perhaps a reputable merchant who can trade your wealth for domars, or a trading post where wares from distant dimensions can be found. From the standpoint of an adventurer, such sidings may be even more interesting than the "main parallels".
Those main parallels, one might conjecture, are the focus of the various worlds of the (A)D&D game. Oerth is certainly one, and I daresay Toril is as well. Surely the other worlds are, too. Naturally, each such world has its own set of inner and outer planes; we see this clearly at the end of the Gord the Rogue novels (spoiler in invisotext; highlight it to read) where Gord finds himself in a new multiverse complete with a new set of outer planes, the ravages of Tharizdun against Oerth (one possible Oerth, perhaps) contained in a pocket multiverse all to themselves. It also explains how the Forgotten Realms has planes (and inhabitants thereof) similar to, but different than, those in Greyhawk. Certainly some places, such as the domain of the Catlord, Weird Way itself, and various other extra-planar locales float above (or perhaps more accurately, between or within) such probabilities.
I think the Way is a fantastic means to shake up a campaign. Certainly it could be used to move from one to another; simply make a jaunt on the Way and the PCs are moved from Athas to the Viridistan. But I think its best value is in adding variety and spice to a long-established campaign; a hidden world known only to a few elite travelers. Perhaps a certain merchant has a reputation for importing impossibly exotic merchandise. Perhaps a newcomer to the Foreign Quarter is of a species never before seen. Perhaps a certain albino with a rune-carved blade is brooding in the corner of the bar...