1. Maps - The 15' Green Trails maps are great if you're staying on trail. If you're planning on leaving the trail it's best to have 7.5' USGS maps. Both kinds are available at most sporting goods stores. Keep your maps in a plastic bag to protect them from the rain and know how to use them.
2. Compass - A map is no good without a compass and a compass is no good if you don't know how to use it. It doesn't matter how fancy your compass is, but if it doesn't have a compensation setting for true North make sure you know how to convert magnetic to true North. Remember in western Washington magnetic North is 20-22° east of true North.
3. Flashlight/Headlamp - A light source is vital if you get caught in the woods after dark. Also carry spare batteries and an extra bulb and make sure you test your light before each trip. Batteries have a limited shelf-life, and contacts can become corroded blocking the flow of current.
4. Extra Food - At least one extra day's worth. It should be something that stores for a long time, requires no preparation and is high in energy. Many people choose things they really dislike so they won't be tempted to break into their emergency rations unless they really need them.
5. Extra Clothing - Bring an extra set of whatever is appropriate for the time of year including warmer clothes for an un-anticipated overnight stay. Don't forget rain gear even when the skies are clear.
6. Sunglasses - Sunglasses are a must if you will be traveling on snow. Snowblindness is a very painful and debilitating condition that requires emergency evacuation.
7. First-aid Kit - Don't just have a first aid kit, have a useful first aid kit. If your kit just has a few bandaids and some aspirin, you won't be able to do much. Make sure you have the supplies to deal with major injuries, and make sure you have the knowledge. Take a first aid course from the Red Cross or the Mountaineers.
8. Pocket knife - That cork-screw just may come in handy someday.