"Meccha" is a good one. This means"really..., freakin...," You put this before adjective. (ex): "Meccha Oishii !!"(It's really delicious!!) "Meccha Tsukareta!"(I'm really tired!) Me and my friends use a lot. Also it's really common among Japanese people, mostly in young people!

Please give more hints what you are looking for. Japanese slang is so rich and abundant!