Equivalent Exchange is one of the many mods included in the Tekkit Pack.
The Equivalent Exchange Mod is based around alchemy/magic. The mod adds in a number of extremely powerful end-game items that require large quantities of diamond to make, and large quantities of fuel items to power such as glowstone, redstone, coal and more. It also grants the ability to easily condense materials into more valuable forms.
It is important to note that in order for Tekkit to work correctly on a server, the server owner must op the mod itself, by typing '/Op [equivalentexchange]'. This is because the mod needs permission to interact with the environment.