"Johnny Works with One Hammer," perfect for a construction themed storytime (pair it with "Five Little Nails"). Or anytime you want to get a little bit silly. The Wiggles popularized this song so the odds are good some of the kids will know it.
"Chester Have You Heard About Harry" is another song I learned at camp in elementary school. My public school sent all the fifth graders to a sleep-away camp in the fall for 2 nights as a field trip, and one of the activities was singing this song over and over and over and over again. There are lots of different lyrics. The version I learned is similar to this video except that I learned "I hear he knows how to wear a rose" but I think "touch his toes" is actually better. I've also heard "how to wear his clothes" but we don't need people picturing a naked guy while we're trying to talk about the library, so I think that's out.
"Sleeping Bunnies" How did I miss this one growing up, in library school, and until now? Thanks to the Storytime Underground crowd for mentioning it on Twitter. It's awesome! The kids love anything where they pretend to be asleep and then "wake up" and go bananas! This version includes other animals and you can make up your own verses too. My friend Miss Mary has a video where she shows you how to play this song on the ukelele, if you are not incredibly untalented musically like myself.