Have you ever wanted to play an older version of Minecraft?
Do you remember the days before xp points, hunger and The End?
Remember when you had to walk everywhere and could make logs burn forever?
Remember when a burning forest was something to be feared and fled from, and being lit on fire was almost certain death?
MCNostalgia is a tool that takes binary difference patches and applies them to the latest version of Minecraft.
It essentially reverts it to an older version, without ever distributing any .jar files illegally.
Binary difference patches are made by taking the differences between two files, one is the source/old file (in this case the latest version of Minecraft), the other is the output/new file (in this case an older version of Minecraft).
With this mod installed, you simply select an older version of Minecraft you would like to rollback to.
This is very useful for playing with older game mods which are no longer made.
You can find this useful mod at:
http://www.minecraftforum.net/topic/800346-tool-mcnostalgia-211-now-works-with-125-download-link-back/