Difficulty Locker allows you to lock the difficulty of multiple worlds separately in versions prior to 1.8 (more specifically 1.7.2 & 1.7.10), where this functionality has been built into vanilla.

I made a huge derp on R-1.2, Update to R-1.2.2 to make sure you don't crash when joining servers!!!

For downloads from different versions check the tab Files

FAQ:

What does Difficulty Locker do?
This mod adds a lock button to your game options screen that when pressed on will lock your world's difficulty and will only be able to unlock again on change of the mod's config.

HELP I can't change the difficulty of Minecraft when no worlds are loaded!
I did this on purpose since the button had no use anymore.

I am a mapmaker and I want my map to lock to a certain difficulty for everyone who uses it.
Since R-1.2 the locked files get saved in the world's folder under the file name "locked", this file specifies what difficulty the game will be locked to on opening of that world.

I found a bug in the mod, where can i report it?
I created a repository on GitHub to keep track of the bugs in this mod, you can find it here:
https://github.com/MattsMc/Difficulty-Locker/issues

Where can I help out translating this mod into various languages?
On the GitHub repo i added the lang files, please translate and then change the file name to your corresponding country.
https://github.com/MattsMc/Difficulty-Locker/