They are pretty predictable. I just tried !osm madagascar on a whim, and it opens OpenStreetMap, as expected.
Setting DuckDuckGo as the search engine for your browser's address bar means all these bangs work right there whenever you open a new tab.
Usually if I need a certain engine I just guess the bang and it's usually supported and correct. !gm for google maps, !tineye for tineye, !wayback for the wayback machine...