I remember reading about this sort of site a while back, but I never realized how convenient and cool it is until I downloaded it from the AppStore.
Swackett basically gives you an idea of if you should be wearing a sweater, jacket, or coat (and sunglasses or a hat). In the drastic inconsistency of San Francisco weather, that means yesterday it was suggesting a jacket, and today, it says I should be dressed as a trekkie:
Nothing fancy, just usable. You don’t have to even register unless you want to manage multiple locations.
This would make a great iPhone/iPad app also. Download swackett from the Mac AppStore