As such, there are three major types of mods; Mods that add places to go, mods that add creatures to encounter, and mods that add gear to help you along the way.
Biomes O' Plenty, Ridiculous World, Ruins, Diversity, Hardcore Ender Expansion, farlanders and Doomlike Dungeons.
Primitive Mobs, Elemental Creepers, Elemental Witches, Dragon Mounts, and Enderzoo.
Quiverbow, Ropes +, Archimedes Ships, Balkon's Weapons, Buildcraft, Better Furnaces, Progressive Automation, Redstone Paste, Tomahawk, Lootable Bodies, Better Storage, and Craft Guide.
There are also some visual and performance related mods...
Shaders, Optifine, update checker, and Java7checker.
-As a side note, you'll have to get your own shader packs if you want to utilize the shaders mod. Not everyone's graphics cards are the same, so I didn't include any by default.
Don't worry though, they aren't any harder to install than resource packs. ;)