Equivalent Exchange is one of the main mods included in Tekkit Classic.
It is based around alchemy/magic and the mod adds in a number of extremely powerful end-game items. These require large quantities of diamond to make and large quantities of fuel items to power such as Glowstone or Redstone, to name but a few. It also grants the ability to easily condense materials into more valuable forms.
It is important to note that, in some cases, in order for Tekkit Classic to work correctly on a server, the server owner may need to op the mod itself, by typing '/Op [equivalentexchange]'. This is because the mod needs permission to interact with the environment.
A re-release for Minecraft 1.7.10 is used in Tekkit Legends. A remake of Tekkit Classic