Maps. There are RPG and GM that can do without, but most I know need some. Especially games that play in a post-apocalyptic universe of one kind or the other need maps, and the GMs are often in need for some nice, cheap urban maps. And for those who don´t want to edit some google-maps, I have another suggestion.
One of my other hobbies is an online game called “World of Tanks”. I don´t play often, I am not really good at it, but I like it. You don´t need to like this game, but you might like the maps. The maps are large enough to be useful and since these are battle areas, they often have a “ruined look” to it.
Just have a look at the following picture-links, perhaps you find something that you like:
If you want to search for further maps, just type “World of tanks” and “maps” into your favorite search engine (I use duckduckgo.com).