Oh, for those of you that like alternate dimensions, check out the
Twilight Forest.
It's basically a huge forest. In perpetual twilight. The 'sea level' has been lowered to make room for the enormous trees that dot the landscape, stretching out far above the other, still rather tall, trees. There are also bigger, branched versions of the huge mushrooms. Glowing or chirping bugs are sitting on tree trunks all over the place, and the forest is populated with various deer, boars and those curly-horned sheep things.
Here and there are hedge mazes filled with treasure chests and small spiders, and there is a large boss arena you can go battle a large boss in. There are also large glaciers to be found, and 'hollow hills' which are filled with new mob spawners and lots of resources. There are also lots of little ruins and some underground mazes all over the place, with more new mobs inside.
My favourite part other than the forest itself are the huge wizard towers filled with winding staircases, libraries and I think I saw one with a small desert inside. They're complete with little side-towers and little outdoor walkways between them.
You can get there by surrounding a puddle with flowers and tossing a diamond inside. If you use Redpower, the new ores and gems still generate properly so you could move in permanently. (Except if you want volcanoes and rubber trees, but I don't like their current form anyways)