Here are some links to pages — mostly Wikipedia articles — on some famous conworlds:

• The World of Middle Earth by J.R.R Tolkien
• The Dune Universe
• The Star Trek Universe
• The Star Wars Universe
• The world of Westeros, Sothyros and the Free Cities by George R. R. Martin
• Mark Rosenfelder's Almea
• The Discworld of Terry Pratchett