Cats should not be allowed to roam outdoors. The can wander into the streets and get run over. They can be picked on, injured or even killed by mean humans. The can be injured or killed by predators - dogs, other cats, or other creaters.
If you really want your cat to go outside, take him/her outside on a harness. Kitty won't like it at first, but puppies don't like leashes at first either. They will adjust.
Or build your cat a nice outdoor enclosure. Something with a top so they can't climb out. If you want to get really fancy, build the enclosure around a window or a door and put in a little cat door so you cat can go in and out at will and stay safe.
Think you can't afford an enclosure? Then either keep the cat inside all the time or find way to come up with the money at http://fuzzychildren.blogspot.com/2009/08/if-you-cant-afford-vet-care.html
References, pictures, ideas
http://habitathaven.com/
http://habitathaven.com/catalog/8_Photo%20Page.pdf
http://habitathaven.com/catalog/10_Page%2010%20WITHOUT%20FLOOR.pdf
http://habitathaven.com/catalog/11_Page%2011THREE%20SIDED.pdf
http://www.catsofaustralia.com/cat-enclosures.htm
http://www.classypetshop.com/cat-enclosure.htm?gclid=CJDz4u7z1ZwCFchM5wodX1aVQQ
