Dartmouth doesn't give A+s but does give "citations" in exceptional cases, which shows up as A* even though it doesn't count for more in the GPA (literally, it's an asterisk...). The prof also gets to write a little comment about why you're so special that gets attached with another sheet of paper to your transcript (though I don't know how AMCAS would handle that 😛).
Edit: But then, those cases are very, very rare. Like how many profs never give even As, even more profs never give citations, or don't even know that option exists. I'd say maybe the prevalence of them is maybe 1 in 500 (students in any class) or even more like 1 in 800 or something, which isn't actually as rare as you'd think considering people tend to take at least 35 classes before they graduate. I've known people with up to 7 or 8 (Valedictorians/Rhodes), and people absolutely and completely thrilled to even get one.