Moved from Sheetweb and Dwarf Spiders.

"It's definitely not E. dentosa. E. atra and E. dentigera do not occur in Washington. It could be E. capra, which replaces dentigera in the West." ~ Rod Crawford

Wow! Excellent shot! Take a look at those distinguishing features:
1) denticulate carapace margin, 2) lack of carapace modification near the eye region, 3) basal femoral apophyses, 4) dark coloration and small body size. All these point to Erigone, specifically either Erigone atra, Erigone dentigera, or Erigone dentosa. Erigone hydrophytae has these characters as well, but the projections along the carapace are larger and farther apart than your specimen.

Grab a good lateral shot of the palp and we could narrow it down to species.

-M