finder
I have a Finder window open to a directory that contains files. They have an associated application to them.
What key combination can I hit to have the selected file opened in its default application?
What's the equivalent of double-clicking that file with the mouse?
I don't want to have to take my hands off the keyboard.
I have tried without success:
Enable show path in the view menu (not sure about the menu names but it should appear at the bottom of the finder window) then cmd + double click the folder you want to open, in this case, the last folder is your current folder.
I made an image but can't post it :\
Edit: Image example 
From this Apple Support article it's clear that the Finder Preferences are overridden by the preference in System Preferences > Dock > Prefer tabs when opening documents.
This seems oddly clunky to me. Apple's GUIs are usually thought out very carefully, but they seem to have made a mess of this. (Well, unless I'm missing something.)
Best Answer
Cmd-O opens the file in its default application.
Also, Cmd-P opens the file and starts the print dialog (not supported by all applications).