Yet another awesome share from my Gandalf.
Planetocopia is one of the best complete world(s) sites I've seen because the maker, Chris Wayan, doesn't just do it because he loves it, but he can explain in great depth why he does it and loves it. His worlds are crafted with great precision, and if there is a hiccup/hole, he can account for it. Whether you're after theoretical worlds, ancient, future, would-be Earth, or even alien worlds with multiple moons, this has them all!
I especially like Jaredia because it's based entirely on the concepts of Guns, Germs, and Steel a book I meant to read for years and years. It was suggested by professors and friends alike, and wouldn't you know, as soon as I bought it and started reading...the 3 part documentary version was loaded on Netflix. Win, win.
This wondrous resource pairs especially nicely with the donjon fractal world generator because it handily explains the whys, hows, and pitfalls of the generated world maps.
If you're like me and "grounded" fantasy on one Earth-like world suits you, or whether you want a smattering of worlds for a vast Traveller system, this will serve your needs.