ME! I am a plumber and as such, I crawl under houses all the time. I have encountered lots of small animals and reptiles in my day, some of them feral cats. Don't be afraid of traps--sometimes they are the kindest thing to use. You can generally get live traps from a vet, fish and game, or the humane society. Our humane society has a program where they neuter feral cats for nothing or almost nothing.
If your crawl access is outside, you can also try to lure the animal out with food. This will entail someone going under the house with bits of food and making a trail out of the crawl space with the food. There is always the threat of an attack if someone does this (so wear heavy clothing and gloves), but it is rare. Generally the animals are more afraid of you than you of them, but remember a cornered anything is dangerous. You will have to stay just outside the access and watch and close the access as soon as the animal is out.
If you do not know how the animal got in, walk around your home and try to find an entrance point. Otherwise, the animal is likely to come back to its nice warm home where there are probably mice and other small rodents that offer plenty of food.
