Basically, if you move to Seattle, you are pretty much restricted to working for Microsoft or Amazon and that's it. Period. Nothing else.
It's a huge career limiting move, make sure you're okay with it.
Google has a big campus here in Fremont and Kirkland, and they're building a huge new complex in South Lake Union (Amazon's stomping grounds).
Facebook just opened their brand new office in SLU.
Samsung is here. Adobe is here. Atlassian is here. Uber is here. DocuSign is here.
Expand the metro area to Bellevue (since we included Microsoft above) add companies like Redfin, Zillow, Expedia, FICO, Disney.
Edit: Oh yeah, forgot Intel and EMC as well.
I'm sure there's many I missed but there's WAY more than just Microsoft and Amazon.
... Just don't tell anyone. Go to Portland instead. I hear they want you ;)
There ARE other cities with a lot of quality, diverse tech jobs outside of the Bay Area. I live in one of them and I don't consider it a 'career limiting move'.