The World Dream Bank is a great resource for people wanting develop alien worlds or alternate Earths. It provides several well-reasoned planets with maps and texts, including terraformed Mars and Venus. Link — >>>
The example below is a future Earth where the polar icecaps have melted with an accompanying rise of the oceans — useful for postapocalyptic campaigns. Click on the map for a larger version.
The Dream Forge 
I’ve enjoyed Planetocopia for a long time. I want to see more of Libratia and I’m looking forward to Lateria.
Libratia ser jättespännande ut. Skulle passa bra i rollspelet Sci-Fi!