When I went outside to walk my dog, I started to slide. Later when I tried to drive my car to the UW, it started to slide and the traction warning went on.
The ground was really iced up in places right now and there are two possibilities of the origin: freezing rain and cold rain on a freezing surface.
Right now the National Weather Service has a freezing rain advisory out for the south Sound and around Puget Sound away from the water:
Freezing rain occurs when rain falls from an above-freezing layer into a surface layer that is cold enough to supercool the rain to below freezing. When it hits a cold ground the rain freezes immediately you can get freezing rain. But according the temperature observations above Seattle (see below), there is no real evidence of much as subfreezing layer above the surface.
There may have been more of a cold air layer at low levels over the south Sound and along the eastern slopes, so classic freezing rain might have occurred there.
But I think it is a bit more complicated that just freezing rain. I suspect what has occurred in many locations (like Seattle) is that with clear skies last night, the surface cooled to below freezing (see min temps below). Then cold rain came in this morning and froze on the cold surface.
Update: there were a number of bicycle accidents this morning on Seattle's Burke Gilman trail due to icy conditions.