There are not many.  Most people learn their lesson after 4 under 3.

http://blisstree.com/live/5-under-4/
http://marzomemories.blogspot.com/2008/04/5-under-4.html

Close but not quite crazy enough:

5 under 5

4 under 3 (no fair; they used actual twins):

http://4under3.blogspot.com/
http://twitter.com/4under3