How can I turn mods on and off with Modloader without uninstalling them?

I keep several different .jar's in my .minecraft folder for different mods. ModA.jar works fine, but when opening ModB.jar, Modloader crashes because it's trying to load Mod A.

How can I tell it to ignore that mod (for now)?

Inside .minecraft/config, there is a file called Modloader.cfg.

Here's what mine looks like:

#ModLoader Config
#Wed Feb 15 13:42:15 EST 2012
mod_IC2=on
mod_Planes=on
mod_SpawnerGUI=on
mod_MAtmos=on
mod_TooManyItems=on
mod_BuildCraftTransport=on
mod_BuildCraftFactory=on
mod_WW2Guns=on
mod_BuildCraftBuilders=on
mod_mocreatures=off
mod_BuildCraftEnergy=on
mod_TwilightForest=on
grassFix=true
loggingLevel=FINER
mod_MineColony=on
mod_ClayMan=on
mod_BuildCraftCore=on

Yep, it's that simple. Just change on to off and vice versa.

Simply rename the ModB.jar to something like ModB.jar.notnow.

NB: You can rename it to [1] ModB.jar to have it loaded before ModA.jar which may fix your problems...