All the presidents of the United States, except Martin van Buren, are descended from King John of England — as shown by BridgeAnne d’Avignon, a seventh-grade student in Watsonville, California. You can buy a poster at we are all related. Via quora and metafilter.
This is less impressive than it seems as first glance. (But still impressive; it’s easy for me to sit here and criticize.) All but van Buren and Eisenhower are at least partially of British descent. That’s actually a larger number than I expected, and implies some combination of:
- people of British descent are more likely to be president, or
- a very large proportion of Americans are at least partially of British descent.
On the other hand, as I mentioned a couple weeks ago, if we go far enough back everyone has a common ancestor. I wouldn’t be surprised if someone starts digging back through van Buren’s family tree and digs this up.
God plays dice 
Do enough digging, and most people with English ancestry will probably find William the Conqueror somewhere in their family tree, and most people of European ancestry will likely find Charlamagne somewhere in there. The gene pool was more of a puddle back then.
According to this source, John Lackland, King of England is Martin Van Buren’s 22nd great uncle:
http://www.geni.com/path/Martin+Van+Buren+8th+President+of+the+USA+is+related+to+John+Lackland+King+of+England?from=353493399110004524&to=4924870419470035934