The five little ducks song- but i usually start with 10 little ducks and count down- if he is unsettled i just keep going and going and if he settles quickly i skip a few numbers. i have also made my own words up and changed "ducks" to "trucks" so its 5 little trucks went out one day- over the bridge and far away - mummy truck said honk honk beep beep - but only 4 little trucks went to sleep" and so on....
I remember doing that song backwards (counting up) one night with DS1 and getting to 80 ducks!

I sing Elanor Rigby and Ob la di. Ob la da - 'cause I know all the words.