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In southern Califronia, it might be a daddy long leg (cellar spider) or it is possible it is a brown widow.

http://www.google.com/search?hl=en&site=imghp&tbm=...

That is about the only fairly dangerous spider that I can think of that might match your description. They are very non aggressive

Steven Y have been given it perfect, that's a species of mouse spider. a number of them could have an extremely painful chew, yet do no longer require scientific interest. additionally they, as you may tell via the photos, have super and effective fangs. they have been regularly occurring to pierce boots.

Could be a lot of things, likely harmless. A photo would really help with an ID. No, don't kill it.