Wow, compared to most of the prior posters, my collection is *much* more modest, but I *am* proud of my effort to squeeze much needed storage space into a basement wall alcove behind the drop-down backdrop covering the wall behind my layout, just big enough (barely, for now!) to hold the rolling stock not currently in service on either the main layout, or my test track.
The first shot was during construction and before I rehung the backdrop (the picture visible behind the shelving is a paper mock-up I printed earlier and pieced together while designing the backdrop, which I attached to the wall before installing the shelves):
The second shot is with the backdrop rolled down over the shelving:
Construction of the shelving was very simple -- 1X4 dimension lumber, with two grooves cut into the top of each shelf to fit the rolling stock wheels, and small vertical dowel sections at each shelf end to keep the items on the shelf, with vertical 1X2 strips screwed behind the shelves and secured to the wall. Here's a detail, from before I added the background and relocated the sign:
