When I was a sophomore in college, I was taught the most lethal and hilarious revenge tactic. You need a few things: apathy(antipathy) towards target, petri dishes, bucket with lid, ability to shut the hell up and not tell a damn soul. This really works best in the winter or in hot areas. It also only works in rooms with rugs and decent gaps between door and floor. It will work well if the target is an idiot and leaves their keys hanging on a hook while showering or leaves the door unlocked while showering or hanging in the lounge.
In the morning, pee that first smelly, yellow pee into bucket(it helps to have a big meal of asparagus the previous night). Close bucket. Repeat next morning. On the thrid morning, if the pee in the bucket does not smell horribly foul, urinate in bucket again. Be careful as smaller buckets are not advisedd for this task. Perhaps I should have made that clear earlier. Oh well. As soon as the bucket of pee smells truly piss-like, get some latex/vinyl gloves and fill the thinner part(the lid) of the petri dish with the smelly pee. Place in freezer. Two discs is recommended for the average room, perhaps three. Four if you are really pissed off.
48 hours later, your pee should be pretty well frozen. The goal now is to take the discs in the petri tops over to the target's room (oh yeah, by the way, never do this to someone on your floor or, really, your building) and place the frozen discs upside down on the floor in front of the target's door. Gently tap the petri dishes so that the dish part will lift away from the frozen disc of urine. Then slide the discs under the door. For obvious reasons, do not attempt while the target is in the room. Since you will have done your homework and know the target's patterns, you know when he/she will be gone for several hours. This will allow the discs ample time to melt in the heated room. If the room is open and you are certain you can get in and out without being seen, leave the discs under beds and/or dressers, etc. Someplace out of sight. The discs melt and a foul smell exudes from the carpet. This will force the target to endure a horrific stench until they have the carpets cleaned.
Unfortunately, the piss disc has only gotten to the 'frozen disc of urine in your friend's fridge' stage in my life. However, it was absolutely hilarious. I wish I had been there to see the expression on his mom's face when he explained what was in his fridge. Sorry, bud.