What's an simple and easy mod pack for minecraft?

There are lots, and lots, and LOTS of mods for Minecraft. What do you want to add?

Amoungst my favorite mods are:
NEI (Not Enough Items) - which helps with recipes.
InventoryTweaks - Item sorting in your inventory and chests.

A simple(ish) mod that doesn't add too much:
Tinker's Construct - This mod allows you to create, enhance, and repair custom tools. Initially these tools will be less useful than their vanilla counterparts, but once you get into metal tools, they are far more capable. It is well documented in-game, as you start with a well-written how-to book.

These are high-complexity mods, but are really worth it.
BuildCraft - Many machines to automate resource gathering and production.
IndustiralCraft - Many machines to automate resource gathering and production.
(I prefer the machines of BuildCraft, but the tools of IndustrialCraft).
Thamucraft - This mod adds a research table, but everything you can do in the above two mods you can do in Tamucraft, but with magic instead. It is also very well documented.