Everyone seems to know this one:
I see London, I see France
I see _____'s underpants!
This is well known enough that if someone's undies are showing, you can pretty generally just say "I see London" and they'll be alerted.
One variation I heard exactly once as a kid, but has been recorded by plenty of folklorists, was to add on:
May be purple, may be pink
I don't know, but they sure stink!
Funny as I thought that was, it always struck me as a line that made no sense (even by playground rhyme standards, which are awfully loose where sense is concerned). If you can SEE the underpants, how would you not know whether they're purple or pink?
A fairly common variation is
Teacher teacher, I declare
I see _______'s underwear
A variaton of THAT is to I see _________'s bottom bare. This is somewhat rare (as are bare bottom sightings compared to underwear sightings, I suppose).
Not sure as to the origin of these yet - guessing it'll turn out to be several decades old, though.
See also: "may be black, may be blue / oh my gosh they're full of poo" which is part of a We Will Rock You parody variation.