A subversion of LOL (laugh out loud), but stands for maniacally laughing out loud. Maniacal can be summed up as "affected with or suggestive of madness". MLOL'ing can be used in gest or when you are simply up to no good.
I just sent you a picture of what I'm going to do to you later on!!! MLOLMLOLMLOL!!!!
- or -
You cheatin' MFer, wait til you see your car!!! MLOLMLOLMLOL!!!!