At some point in your cat's life it's going to pee on something. Whether it's because they got themselves locked in a closet, have some sort of infection, are stressed or are just doing it because they feel like being evil little shits- it's going to happen. Don't say "My cats never pee on anything!" because you'll jinx yourself and they'll do it just to prove you wrong. I know- I said it. In my opinion, there is nothing in the world that smells worse than cat urine. It is the most powerfully foul stench and it can be smelled damn near a mile away.
Last time I changed the litter box I didn't really have enough litter to fill it to the normal level. I figured it would be fine until I got to the store to get more then I forgot about it. Fast forward 6 days and the litter box was getting stinky. I still hadn't been to the store but I was planning on going in the morning. At 6AM I was awakened by the unmistakable stench of cat urine. After coming to my senses I also realized that my leg was wet. One of the little bastards peed in the middle of my bed while I was sleeping in it! I could have killed them both. After showering and stripping the bed and fuming the whole time I was at a loss for what to do about the mattress. So I threw the linens in the washer with a cup or so of vinegar and started searching the internet for the answer. Well I found plenty of articles for so called cat urine neutralizers like Urine Gone and Nature's Miracle but I have already tried those in the past with lackluster results and there was no way I was going to sleep on a mattress with even the slightest smell of cat urine on it. I would get a new mattress first. Then I happened on an article claiming to get cat urine smell out anything using vinegar, baking soda, hydrogen peroxide and a little dish soap. Well I had all of those thing so I set to work somewhat skeptically.
OMG it worked. It's amazing. There is NO smell. NONE. I am so happy. I am really shocked that it eliminated a lake sized urine stain and smell with one treatment and probably about 5 bucks in materials. So I felt I should share this little miracle with everyone because no one should have to live with the smell of cat urine.
Yell, curse and fume about the little bastard who has ruined yet another item of yours. This is why we can't have nice things. It'll make you feel better. Punishing cats is generally ineffective because when the little shits get pissed at you they tend to piss on more things to get back at you. They own you after all.
If the area is still wet blot up as much of the urine as you can with paper towels.
Make a 50/50 dilution of vinegar and saturate the stain and a good way around it with it. If the urine is on carpet a really small spot on the surface will spread out a whole lot underneath and you need to get it all to eliminate the smell.
Blot up as much of the vinegar solution as possible.
Spread a good amount of baking soda over the whole area. Allow to sit for 15 minutes while you continue to curse the cat.
Mix 1/4 cup of Hydrogen Peroxide with a teaspoon of dish soap (scale up if your area is large) and drizzle it over the baking soda on the stain. Work in in with your fingers (rubber gloves if desired) or a brush.
Blot up as much of the liquid as possible then allow to dry completely. (I used a hair dryer to speed the drying process as I would like to sleep tonight)
Vacuum thoroughly to get rid of the baking soda.
That's it. Smell gone. Miracle.
Also, an update- most cats will remark something if it smells like their urine. Well it's been almost 2 years now and it has not happened again so I can only assume they can't smell it anymore either.