Well, both times I've been to Seattle in the summer, the weather was very pleasant... temperatures in the 70's... it was about 80 one day and I was told that was pretty hot! (I'm sitting here in Louisville in June and it was 94 today!!!) Also, it was sunny... both times I was there in the summer it was sunny and beautiful outside. This is certainly the time to visit. So, you probably want some shorts and long pants... some short sleeves and some long ones... for night and for day.
During the rest of the year I'm told it rains very frequently, or maybe just gloomy days. Hey, but gloomy is alright... get a cup of coffee and warm yourself up... you'll be fine! Also, very little snow here in the winter... maybe a couple inches at most.
Well, for most of the year, Seattle has a chance of rain. June-early September it rains very little. So a umbrella or light jacket in spring is fine.
In Winter, it gets chilly and windy sometimes so a heavier coat is needed.
In the mountains during ski season you should use sunblock if its clear, the reflection off the snow and from the sky can give you a nasty sunburn or a stupid outline of ski googles around your face.
Miscellaneous: Sometimes its warm!
Clothing/Shoes/Weather Gear: If you are visiting Seattle during july to september, you should expect to see beautiful sunny weather. You can plan to pack light summer clothes. During the off summer months, (october thorugh march), make sure you pack an umbrella or raingear, and a warm jacket.