Your comments

@Erik, The problem is that many files don't have extensions. Files like Gemfile, Vagrantfile, Berksfile are standard named files that have Ruby syntax but no extension. The solution would be to have a way to list some files and specify their extensions. Bonus points if we could use regex in the filename.
This is actually a duplicate of

As mentioned in the above link, Visual Studio has a great Opened Files dialog (Alt-tab).

I agree. Emacs has this incredibly useful command (Ctrl-x, Ctrl-b) which lists opened files (called buffers) in order of last visit, with extra info like whether the file has changed from disk (edited). We can then move up/down the list with the arrow keys and press Enter to bring it up front in the editor. We can also press "x" to remove the file from the stack list (close it) or "s" to save it if it has changed.

Sublime Text 2 has currently a Go next/prev Stack which is just short of this functionality. There really needs to be a "Show stack" keyboard shortcut to show the files in the editor in a given order (last visited - stack, last modified, alphanum).

I don't think it necessarily have to bring the list in the sidebar. This is a temporary view that is closed as soon as you select the file you want from the stack list. It is like a search window in that sense.