Golden Slumbers by The Beatles - played/sang it to DD all the time when pregnant, and do so now as well. I really think it soothes her.

I also sing a lame little song I made up in the wee hours of the morning to the tune of 'frere jacques', with the lyrics:

You're my baby,
You're my baby,
Baby girl,
You're my world.
You're my little baby,
You're my little baby,
Baby girl,
You're my world.

I put her to sleep with that at least once a day.