Is there a key shortcut in Visual Studio to find & open a file inside your solution without using Resharper or any other tools?
Not a single keystroke, but yes: Ctrl+D > o f
(so that's five keystrokes before you start typing the filename). It then autocompletes the filename for you.
You need the Find box in the toolbar for this to work.
(If you change your mind about addins, SonicFileFinder is for this very purpose, and free.)