Some urban legends are actually based on true events, such as the case of the young man doing target practice on a large [saguaro cactus]?. He was killed when his gunfire severed the trunk, resulting in his being crushed by the falling plant. Even when essentially true, however, the stories are often distorted by the many retellings from the original event.
There is a thriving newsgroup, news:alt.folklore.urban that discusses these stories. The newsgroup's [Frequently Asked Questions] page summarises the truth or otherwise of these stories, so far as this can be determined. A similar list may be found with The Urban Legends Reference Page at [snopes.com]. For online urban legends, a good source is [Virus Myths]; another is the Darwin Awards site, which also showcases a few stories each year of dubious veracity (they've promulgated Urban Legends as facts in the past).