Seattle's last frost date has come and gone, March 27th, with a safe date of April 15th.
In other news, it snowed in North Seattle yesterday! As I scan the news this morning I see pictures like this:
And articles titled "Seattle Gloom: 40 days without much sun"
Ok enough sulking, onto some learning. The average last frost date in Seattle doesn't really mean much. I usually wait until late April to plant outside. Some vegetables that may be able to handle being planted now are pea's, lettuce, beets, spinach, and carrots.
Planting too early will lead to stunted growth, which lowers the plants resistance to disease. Last year I planted pea's in my garden in early April, and 1 month later planted another set. They ended up growing at the same rate! The left was planted in early April and the right was planted around May 1st (these photo's were taken on the same day):