Is there a shortcut to open a folder in a Open File dialog? In an Open File dialog, I can press Return or Cmd-Down to go into a folder. But is there a key sequence to actually select the folder as the item to open? It seems I can only just use the mouse to press the Open button while the folder is highlighted.

(BBEdit is an example of an application in which I often want to open a folder.)
Return (with the ↩ symbol) opens the folder for browsing its contents (unless you're in column view). You need to press Enter instead. It's a different key, and its symbol is ⌤.

If you don't have that key on your keyboard, you can press Fn-Return to achieve the same effect.