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

Constructed Nations

• 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.
• The Republic of Novegrad is a really nicely done con-nation; the website is bi-lingual in English and Novegradian, a constructed language.
• The Armorican Isles