Planetocopia describes itself as: “a group of model worlds supporting intelligent life. Some of these worlds are set in our future, some are alternate Earths, some are purely imaginary experiments in planetology, biology, sociology.”
Looking at worlds like Inversia (Earth, but the seas are landmasses and the continents are oceans), the totally badass Lyr (a beautifully-mapped water world) and a terraformed version of Mars makes me want populate them with my own ideas — or dive into worldbuilding, which I haven’t done in years.
The coolest thing about the site is the level of detail. World maps are clickable, with each section linking to a closeup and a description of that region, and there’s a ton of information about every aspect of the various planets. From my layman’s perspective, the science looks pretty solid, too. (Via Ken Hite’s LiveJournal, by way of anyway.)