The Washington State Digital Archives are one of the oldest and best of the online state archives. Definitely worth a visit.
They (somewhat confusingly) stake claim to more than 85 million online records, 26 million of which are 'searchable'. Not sure what that means, but either way, it's a large, impressive archive that includes the following list of collections:
Mt. St. Helens, Before She Blew...
There's a lot of terrific family history information here, including vital records (birth, death, marriage), census, military records, and much more. A treasure-trove for you genealogy types. The Genealogy Page at the Washington State Library offers even more information, including a free obituary look-up service.
Two other Washington State digital resources of note are:
Digital Collections and Resources, consisting of an assortment of historical newspapers, maps, photographs, military archives, and the usual kowtowing to Governors past.
The Washington History collection probably overlaps totally with the archives already mentioned, but presents particular collections of interest to the history of the state, including a Territorial Timeline, Washington's Rural Heritage, World War I Remembered, and things of that sort.