Catholics Come Home. Watch the short introductory video on how to use their site, but it's really simple: they have tabs labeled "I used to be Catholic," "I'm not Catholic," and "I'm Catholic." Click on the appropriate one to enter in to all the resources they have prepared for these specific audiences.
Personally, I think the best part of their site is the "Epic" commercial. It always brings a tear to my eye as I remember how completely awesome it is to be Catholic!