Recently, I was traveling back to the US through Vancouver, BC, Canada. I was delighted with the fact that I was able to get pre-screened through US Customs before even getting on the plane. What other cities/airports offer this feature? It sure saved a lot of time.
Mostly Canada, but also a little bit in the Caribbean, and Shannon and Dublin in Ireland.
The complete list: CBP Preclearance Locations
It doesn't necessarily save time, but it does allow you to fly directly to smaller US airports that aren't international airports and which don't have customs facilities.