There are a lot of biomes in Minecraft, and a lot of mods that add more biomes. But ExtrabiomesXL is insane. It adds in a LOT of new biomes, and a bunch of new blocks too.
These biomes and blocks are sure to make your Minecraft shenanigans way more interesting.
ExtrabiomesXL is always being updated, and is even supported by many resource packs and other mods. You can read much more on the development blog.DownloadForumInstall Guide