[V1.5_01] Sponges (v2.1) by Exanadu

Backup your minecraft.jar file (always do this before installing any modified file)
Delete Meta-Inf folder if you havn't already in the minecraft.jar.
Make sure you have installed Modloader 1.5_01. Place class files, exanadu and gui folder inside the minecraft.jar

Sponges uses block Ids 252 and 253 by default you can change the block ID number in the exanadu/Sponges.properties file located in your mincraft.jar/exanadu folder

enjoy