"Hide and Seek" by Imogen Heap. http://listen.grooveshark.com/#/search/song?q=hide%20and%20seek%20imogen
I haven't actually tried it during a session, but when I listened to it just now, it sent me into a contraction state that seemed very similar to the one I experienced during my best session to date. The song absolutely gives the chills. I was just sitting here in my chair listening to the song and getting into it, and I found my body forgotten and my anus contracting repeatedly. It was the only time I've really felt anything similar to Aneros use without anything physically there.