"Never was so much misattributed by so many to so few" - Mark Twain
Yogi Berra and Winston Churchill are probably the people most often mistakenly cited as the source for catchy quotes with Mark Twain running a close third (see the above slightly altered Churchill quote misattributed to Twain).
Wikiquote is a great resource for finding quotations but it can be unreliable in its sourcing. Snopes can sometimes be a help in figuring out if someone really said something but this is not its main purpose. Recently THC came across Quote Investigator, a useful website solely devoted to unearthing the sources of famous quotes. It was while searching for a quote usually attributed to George Orwell:
It turns out that while Orwell said things similar to this, the actual quote is only about twenty years old.
"People Sleep Peacefully in Their Beds at Night Only Because Rough Men Stand Ready to Do Violence on Their Behalf"
The site is searchable by using the names of those to whom a quote is usually attributed and the discussion is usually very enlightening. It is less about debunking and snark than illuminating the often complex process by which a quotation can derive from several sources.