The Prime Material Plane (PWH)

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Though planars look down upon primes and their clueless ways, the Prime Material Plane cannot be completely disregarded. It's as wondrous and infinite a plane as any, with an endless variety of places to explore. A countless number of prime-material worlds float within separate vast crystal spheres, which are themselves contained in an ocean of fiery material called pholgiston. Isolated as their worlds are, most Prime inhabitants believe themselves to be at the center of the multiverse, and their world the only one in existence.
And here's a word of caution to arrogant planars: Each Prime world is a unique domain, with its own environment and inhabitants. Traveling to the Prime without information about the destination is akin to casually jumping through a portal into an unexplored layer of the Abyss and expecting to survive. That said, here's a look at a few of the more prominent prime-material worlds and their inhabitants.
==Aebrynis==
Hardly worth noting is Aebrynis, a world with very few portals or sites of interest for a planewalker. Because of a few local powers' supposed demise, a number of heroes of this world have special abilities related to their bloodlines and they rule by this birthright. Aebrynis has connections to a place the natives call the Shadow World, which is probably a demiplane (unrelated, most likely, to the Demiplane of Shadow except in name).
==Athas==
A secluded, hard-to-reach world called Athas is notable for a couple of reasons. First, "hard-to-reach" isn't just hyperbole; portals leading here are rare in the extreme, and spells that allow interplanar travel fail more than half the time. Recently, the githyanki attempted to reopen one of the few permanent portals to this world from the Astral, but their efforts were thwarted by the natives—which says a good deal for the inhabitants' might. Also, while the priests here have interesting connections with the Inner Planes, they refer to a number of the Paraelemental Planes by different names—so don't be confused if the sods talk about places called the plane of Sun or Rain. 
==Krynn==
Krynn is a world preoccupied with its own problems and not very active in regard to the planes. In fact, the inhabitants of this world refer to practically any plane other than the Prime as the Abyss, so they give new meaning to the term "clueless." Aside from a few new spells and magical items, there's not much here of interest—although ruins under the Blood Sea of Istar are said to contain great treasure.
==Mystara==
Mystara is not as active as Toril or Oerth in the planar sense, but it's a world filled with interesting magic, particularly in a realm called Glantri and in another fallen empire called Alphatia. Both have their share of gates and planar accesses, but most of the inhabitants of this world are too self-absorbed to leave for long (at least on purpose). 
==Oerth==
Oerth was the original home of the lich Vecna, who now lives on the plane of Ash, in a citadel bordering the Negative Energy Plane. This Prime world is of special interest to a number of powers, not the least of whom are the drow goddess Lolth and Iuz, a demipower who originated here. The chant says that there once was a wizard here so powerful (and foolish) as to temporarily imprison some Abyssal lords (including Graz'zt), a proxy or two, and even some powers' avatars. Not surprisingly, no one's heard from him in a while.
The world itself has a few permanent gates, but native heroes attempt to destroy or lock them when they can—the people here have a healthy fear of planar beings. A few that might still be open include gates to the main Elemental Planes in the Temple of Elemental Evil, a conduit to the Abyss found within the Vault of the Drow, and a few portals to the Lower Planes in a ruin called Castle Greyhawk.
==Ortho==
Still think the Prime has no interest or influence on the planes? It's this world that gave Sigil the Harmonium (for good or ill)! About 500 years ago, a bunch of cutters here decided to "rid the country of chaos and bring peace to the land." Well, they succeeded, and went on to reshape their entire world in the Hannonium's image. Not satisfied with that, they moved on to the planes, carrying their message of harmony through law with them. Now the Harmonium's one of the most important factions in Sigil, responsible for maintaining the peace and policing the city. The Harmonium maintains a strong relationship with its prime roots; in fact, the current Harmonium factol was born on Ortho. Needless to say, Ortho embodies the Hannonium ideal of peace, harmony, and law—berks who can't stomach these ideals (chaotic planewalkers and free spirits, mostly) should just avoid this place.
==Toril==
Of all the Prime worlds noted here, this one's definitely of the most interest to planewalkers. For one thing, it holds gates and portals to virtually every plane. An elven island called Evermeet's got gates to Alfheim (on Ysgard) and to Arvandor (on Arboreal. Some berk of a wizard has a number of open portals in an underground place called Undennountain that lead to the Lower Planes. (Traveling between the Lower Planes is possible by going through this labyrinth—but it's not recommended.) And an ancient realm called Netheril had a bunch of portals and gates to most of the planes. Though that empire's long gone, some of the portals still work—problem is, their locations on Toril have been lost. The point is, there're more portals here than a body could catalogue in a lifetime.
Here's the dark of it, though—this world has a lot of high-up wizards who don't twig to planars popping in and using their gates. They've got no particular respect for planars (especially those with attitude), and they've the power to back up their own arrogance. (In fact, a number of them are noted planar travelers in their own right.) Further, they've got their portals pretty sodding well protected. But there's no denying that Toril is one of the most magic-rich Prime worlds, so it's a great place for a planewalker wizard to learn a few new spells or gather a magical trinket or two.
A whole pantheon of powers have an affection for this world, and their avatars commonly appear to its inhabitants. Supposedly they use a big staircase from a mountain called Mount Waterdeep, which rises up to some planar nexus point, probably somewhere on the Outlands. 'Course, why the powers would even need to use something as mundane as a staircase isn't something the natives willingly think about.
Tarsheva Recommends: On Krynn, a tavern called the Inn of the Last Home is known for the remarkable hospitality of its two owners, Caramon and Tika. It's said that some of the world's great heroes spend a good deal of time there, but planewalkers have little time for local luminaries. (Tarsheva's addendum: Here's the latest chant. Recent upheavals on Krynn—involving native powers and invading fiends—make this an unstable world to visit at best. Chances are, a planewalker popping through a portal'll be taken for some leftover fiend and slain on sight. Just goes to show that things on the Prime sometimes change as quickly as they do on the planes.)