There's far too many spiders to say which species it is for sure, you would often need a specimen to determine it by shape of the 'palps' in a male which this is, or the 'epigynum' of the female unless it has unique features.
http://www.mnh.si.edu/highligh...
It could be in the Family Amaurobiidae ..
http://bugguide.net/node/view/...