Pre-made rain cap for plumbing vent? (Preventing rust-through of galvanized pipe in the walls)
I used a quick and very cheap solution, taking a standard plastic elbow and jamming it on to form a vent cap:



This will not affect vent operation, but will keep a large fraction of the rainwater out of the pipe, potentially extending the pipe lifetime by a few decades.  This also has a presumably minor benefit of keeping rainwater out of the sewer.