Sometimes you are working along in DEVONthink and realize you want to check out something in the same group, or maybe a group in the same hierarchy. Command-click the name in the title bar of a document window to show a dropdown of the hierarchy the file resides in. Clicking one of the locations will open it in a new window leaving your current window open behind it.
Note: This does not work in fullscreen as there’s no title bar to click on. If you are working in any view where you can see the database’s contents and the contents of your file in a editable window, it only works when your focus is on the preview/editing pane. And this trick works in most other document-based Mac apps, too.