Highlands

Features:
44 new biomes
14 new trees
A variety of useful small plants
Singleplayer and Multiplayer
Highlands and Highlands Large Biomes World Types
biome-specific ore generation (some biomes have more ores!)
All trees can be grown with saplings and bonemeal
256 block high Mountains
More realistic biome placement
Villages in more biomes (now with config file)
Better oceans and islands
Tons of options to customize
Uses Forge

Forum thread for more info/pictures

http://www.minecraftforum.net/topic/2729195-forge-highlands-22/

Note: Highlands 2.2.1 (and later) 1.7.2 versions also work in 1.7.10, as Mojang changed none of the code I use.