Common Words:
* downfall
* downhill
* downpour
* downright
* downtown
* downward
* downy
Less Common Words:
* downcast
* downdraft
* downhearted
* downplay
* downscale
* downshift
* downstream
* downtime
* downtrodden
* downwardly
Phrases:
* down the road
* down to the wire
* down in the dumps
This list is not exhaustive, and there are many more compound words that start with "down".