This fourth edition scenario is very much in the hexcrawl/megadungeon tradition. It consists of a number of sites spread around a valley that can be visited in any order the players desire, and the locations have varying levels of difficulty.
Long ago, a dark power traveled through the space between worlds, intent on finding world upon which to wreak havoc. After seeding countless wars on many worlds, it found a new world to ruin. There it crash-landed, embedding itself deep in the ground and carving a valley-sized furrow in its wake. Patient beyond mortal comprehension, it began to sow the seeds of evil and reach out to those of a perverse and corruptible bent.
The Chaos Scar is a long, wide valley carved long ago by the fall of a massive meteor. As the giant rock passed overhead, milk curdled, livestock fell over dead, and ill fortune befell all. The meteor crashed into the earth with deafening force, and red radiance lit the sky for a week. Then it vanished.
The Chaos Scar itself is death to most who wander in. It is filled with evil and riddled with caves both natural and tunneled by generations of monstrous denizens. The deeper one travels into the valley, the deadlier the foes lurking in its caves and hollows. Strange features have been raised, or have simply appeared, within the Scar—circles of standing stones, bizarre towers, grotesque cottages, and other more otherworldly features. Capping off the valley is the fortress of Hallowgaunt, home to the mysterious Brotherhood of the Scar, crowned by a perpetual storm of black clouds and crackling lightning.
Okay, ignore that last bit about the perpetual storm of black clouds and crackling lightning. The rest of that description is pretty evocative, and though it is aboveground, it fits the classic megadungeon structurally. It still has a bit more linearity than most people interested in sandbox play would probably want, but that is relatively easy to adjust.
There is no overarching campaign goal other than to reach the end of the valley and destroy the meteor. … Finally, feel free to allow the PCs to chart their own course. One of the goals of this campaign is to reduce the workload of DMs running it. Once the valley has been populated by a few caves, PCs should be allowed free reign to choose which dungeon they approach next.
Most of the content is behind the D&D Insider paywall. I was briefly a member a few months ago so that I could see what was on offer. Most of the locations within the valley are not that special though, and are of the form “monster X has been drawn by the power of the evil relic to the valley and has taken over abandoned structure Y”; it’s easy enough to make such areas up yourself, or drop modules in. The published ones also tend to be rather small, and the encounters are not usable directly for editions other than 4E.