I moved to Seattle from Edinburgh. There is a lot of great biking here, but unfortunately most of it has dubious access issues and therefore isn't suitable for open discussion.
If you are after mainly (gap) jumps, Duthie is great. For local XC the standard Tiger ride off the Evergreen page is good. The most readily available extensive XC/all mountain trails that are legal are about 2h north in Bellingham at Galbraith. If you pop into the LBS (called Fanatik) in Bellingham they'll give you a map and some helpful hints.
Sorry that I am unable to be more forthcoming. Wen I moved here it took me ages to understand the weird access issues, especially coming from Scotland with the right to roam act.