I have my computer next to the window, and I live in Michigan, so I'm looking forward to the free cooling.  What I plan on doing is opening the window ~2-3 inches, duct-taping/insulating half of that, and funnel the other half into the side of my computer.  Cut-to-fit screens are optional.

Edit: About precipitation, I'm thinking of curling the 'air duct'  into a bell-curve shape, that way if any water did find its way into the duct, it would just slide back down/out.