Minecraft Mod — MCPatcher HD
This utility patches Minecraft so that you can easily switch between texture packs of any size within the game.
Unlike most mods, MCPatcher is designed to be version-independent. It should work across multiple versions of the game, theoretically as far back as the 2010 Halloween update.
This video walks you through how to install mods with MCPatcher and even provides instructions for Mac users, for whom mods can be more troublesome: