Recently I’ve been experimenting with a tool called Vertex Meadow by Ian Maclarty.  It’s a wonderful browser-based terrain editor that uses height and color maps, meaning anyone, even without 3D modeling skill, can make explorable 3D environments with relative ease.  It features an exploration mode, where you can move freely in your environment, and an edit mode, where you can paint heightmaps in black and white (lighter = higher) and view your results in real-time.  You can also edit attributes such as the height of the sky, the ambient light of the area, fog, specularity, and more.
When you’re done creating your world, you can set a title and then share it, creating a unique URL for your world.