According to Seattle Tilth's Maritime Northwest Garden Guide, here are some good things to plant in September:
Salad greens: small plants overwinter better than tall plants
Cabbage (for spring harvest)
If you're interested in starting things to over-winter (barely eek thorough the winter, but finish up in Februrary or so) you can start:
Grains: Barley, Rye, Spelt, Wheat
You can also plant spring bulbs in late September/early October. This year I'm planning some tulips, crocus, and possibly some grape hyacinths.