Ditto on it depending what your interests and skills are, but I know people who've been happy at PATH, ACRS (Asian Counseling and Referral Service), and Community Health Plan of Washington, and they all pay well for non-profits.
There are lots of wonderful small local organizations in Seattle that keep their employees happy with good work and thoughtful management (and lots that do not!), but the pay is often painfully low. Depending on your level of experience, though, you might prefer to start with one of these as there is often more room to quickly take on new responsibilities and work your way up. Larger, more corporateish nonprofits are more likely to pigeonhole you, so if you start in an entry level or admin position it can be a struggle to transition to a more position with more responsibility.