leet speak
Dumbest fucking thing in the world. Not to mention it's extremely difficult to read unless you're an annoying 13 year old boy or 16 year old girl, and it often takes longer to type out words in leet than it does to regularly type them out.
13 y/o boy: "OMG ur t3h U83R H4XX0RZ!!!1!!! LEET SPEAK IS T3H 4W50M3N355!!!!11!!1!!!1"
Common Sense:"Shut the fuck up, you fucking idiot."