The anatomy of first aid, one of many interesting-looking links from the now-defunct Virtual Hospital.
Medical information for writers of Sentinel fanfic - their so-called 'department of forensic pathology' has zero information on forensic pathology and contains first aid instead. Otherwise, it seems reasonably good.
Triage - a vital concept in emergency medicine.
EDIT: j_rolande mentioned trauma.org in the comments. Very cool emergency medicine site aimed at professionals. The ER doctor game on that site is educational and explains what you did wrong when you kill people (which you'll probably do, if you're not a doctor).