A Large Brown Recluse
Most of the times I am able to get a picture of a brown recluse (Loxosceles reclusa), the subject is already pretty smashed up. I found this one on a bag in my garage and pulled it out on the driveway where I knew it could not escape so I could get pictures. I have been seeing more of these here recently. This one was rather large for a brown recluse. Of course the characteristic marking on the dorsum of its cephalothorax which resembles a violin is the best known identifying feature.
Here is a good article on the bite of a brown recluse spider from Wikipedia. It mentions that an estimated 80% of reported brown recluse spider bites are misdiagnosed and it gives a nice list of infectious and noninfectious conditions that have previously been misdiagnosed as brown recluse bites. I do find this portion lacking in that it does not mention specifically the classic case of an attack from 'The Methicorn'.