Mike and Matt of the awesome ULTRASOUNDPODCAST have released Volume 2 of their brilliant Introduction to Ultrasound iBook.
Whats in it? There is prehospital section by none other than Cliff Reid
Okay its not free but hey not everything in life is free. I highly recommend both volumes. I have used Volume 1 as a teaching aid whilst flying out to a prehospital patient with a colleague to help supervise him doing a nerve block and reducing a dislocation.