Vertex Meadow is a web-based tool for making infinite (well, infinitely looping) vertex worlds, by game maker Ian MacLarty.
Vertex Meadow worlds are created and shared in a browser, and in addition any game is able to be edited (by pressing the 'E' key).
Making the worlds involves drawing with different kinds of brushes, building up and colouring each layer of a collection of heightmaps, and the results are shown in real-time. It's also easy to switch between editing and playing at any time. For more detailed instructions see either the VM homepage, or check out the guides section.