This article is from the Miscellaneous Cat Information FAQ, posted to rec.pets.cats newsgroup. Maintained by Cindy Tittle Moore with numerous contributions by others.
For fresh urine: clean the spot with any good carpet shampoo (Spot Shot is one). Then soak it with plain old club soda, leave it for about ten minutes and blot it up.
If the urine has soaked the pad and the floor below that, it will be difficult to remove the odor regardless of what you use.
To find spots if you're not sure where they are, get a UV lamp that has the filter built in (to eliminate any remnant visible light). Urine fluoresces in "black light." You can get them at hardware stores. There are also UV lamps in hobby stores and places that cater to spelunkers and rockhounds, but they're more expensive. The UV source is safe as long as you use the longwave lamp and not the shortwave lamp used for tanning.
Products that remove odors: Nature's Miracle (carpet, has 800 number); Simple Solution (carpet and other items); Outright! (carpet); Resolve (carpet, perhaps other items); Odor Mute (originally for deskunking dogs, has other applications, leaves white residue, works on concrete). Odor Abolish, by Endosome Biologicals, may also be useful. These products use enzymes to break down the odor causing compounds in urine and feces, and are quite effective.
When using enzymatic products, it is important to use freshly diluted enzymes, let it soak in as deeply as the urine has penetrated, and *keep the area warm and wet for 24 hours*. Chemical reactions, including enzymatic reactions, go faster at higher temperatures. Unfortunately, most enzymatic reactions don't do well much over 102F (38-39C)-- so not too hot. Try covering the area with towels soaked in plain water after applying the enzyme, then a shower curtain or other plastic over that to make sure the area stays moist.
The enzymes in laundry products are the same as those in the expensive odor-killing products, but they cost less than 1/3 as much. They work just as well. Biz is one product. You'll find it in your grocery laundry section with the pre-soak laundry stuff. Remember, you have to soak the area and then cover it to keep it from drying out. The smelly area must be wet with the enzyme for 24 hours or more.
On launderable items: put in the washing machine with a cup of vinegar and no detergent, then wash again as usual.
If you have concrete (eg, in the basement) into which urine has been soaked, this can be difficult to remove, as unsealed concrete is very porous. You will have to neutralize the urine and then seal the concrete properly. A specialty cleaning service is probably the best way to properly neutralize the urine in the concrete. Vinegars and other cleaners may help, but only temporarily. Odor Mute is reputed to work on concrete. Improving the ventilation may also help. In extreme cases, pouring another 1/4-1/2 inch layer of concrete over the original concrete will solve the problem.
Hardwood floors that have been stained with urine can be difficult to clean. First treat with an enzyme-based product such as Nature's Miracle to remove the odor. You can find wood bleaches and stains at your hardware store: you may want to consult with one of the employees on what is available. You will need to remove any varnish or polyurethane from the area, sand it down a bit, bleach and/or stain it, and then apply the protective coat. There are also professional companies you can consult. In severely stained cases, you may have to replace the wood.