Below is the complete list, per the Washington State Virtual Currency Licensee List, that are currently accepting Washington State Residents.
Bitflyer, https://bitflyer.com
Bittrex, https://bittrex.com (not currently accepting signups)
Coinbase / GDAX, https://coinbase.com & https://gdax.com
Gemini, https://gemini.com
Bitpay, https://bitpay.com
Posabit, http://www.posabit.com/
Square, https://squareup.com/
Coinme, https://coinme.com
LibertyX, https://libertyx.com/
Uphold, https://uphold.com/
Xapo, https://www.xapo.com/