The Web is full of imaginary worlds. I’ll try to link to as many as I can, but I will only include “reasonable efforts”, that is, sites that actually have a bit of content.
- Orion’s Arm: A far-future, transhumanist hard science fiction setting.
- Planetocopia: Various alternate Earths, all beautifully laid out, and described in great, scientific detail.
- Life on the Planet Furaha: Life on Nu Phenicis IV
- Virtual Verduria – also has a Wiki
- This Orb – A project that aims to create a science fiction planet in great detail
- Forgotten Futures – RPG Settings based on public domain science fiction settings, like Lost World or Weinbaum’s Sci Fi short-stories.
- Constructed Worlds Wiki – Has various designs.
- The Artifact is a science fiction role-playing game/setting centered around an artificial, roughly egg-shaped world
- Modified Mars – A terraformed mars conworld project. Seems to have been abandoned, but the map is pretty cool.
- Aurora – with most details linked from the Wiki page and the Aurora One nations page.
- Farland – pretty cool conworld, lots of detail.
- Bergonia is a fictional island nation that rests Atlantis-like in the center of the Atlantic Ocean. The island looks a bit square-ish, but it looks like a lot of time and effort went into creating this nation.
- Kingdom of Kaupelan another constructed nation, unfortunately their website is at a free webhoster that forces you through advertisement pop-ups so beware.