I think the closer grain of basswood looks better in models, for the most part. Balsa is great for model planes that fly, as it's light and cuts easy, and for hidden parts of strucures and cars.
Lately I've been saving Starbucks stir sticks for planking.
If you have a fine-furniture refinisher nearby, you can sometimes get fine-grained veneer scraps for free too.
Single-ply cardboard, illustration board (Strathmore / Bristol board), card-stock, and manila file-folders are all materials I used to use for model railroad structures.
Plastruct and Evergreen also make styrene sheets in various scales with things like stone, brick and shingle details molded in.
Edited by Ace-Garageguy, 08 March 2013 - 09:43 AM.