The first meta question asked Should we require references to back up all answer?. The general consensus was yes (most of the time). This is a quote from the top voted answer by @Jez

I think the approach should be that we "strongly encourage" references to reliable web sources in answers

Notice the word reliable. Obviously we can't have our sources coming from our aunt's blog, or Yahoo answers. But what is a reliable source? Published studies are good sources. How about WebMD or Mayo Clinic? Are independent articles written by doctors good?

What do you think some reliable sources are? Should we downvote answers that don't have reliable sources? If you have any good reliable sites, make sure to the community wiki :)