Other than Snopes.com?
Check your library or bookstore for books by Jan Harald Brunvand.
Surely the most entertaining site is The Museum of Hoaxes and the books have added material.
Here is the webpage for the alt.folklore.urban newsgroup. I don’t know how well it’s maintained though. It’s had the same site design since I last visited it about 5 years ago.
Richard Roeper has written a couple.
This one is about general ULs and this is specifically about Hollywood ULs.
I haven’t read either of them, but the reviews on Amazon are generally positive.
There’s quite a few in these tomes.
I like The Big Book of Urban Legends. All of the urban legends are rendered comic-book style. Ones that were already creepy to begin with look even creepier!
My dentist’s secretary’s brother-in-law’s colleague’s cousin’s former friend has proof such a website/book doesn’t exist.