Notepad++ show open files on the left
Settings > Preferences > tab General > Document List Panel > check Show
Settings > Preferences > tab General > Document List Panel > check Show
Credit to anonymous coward
In Notepad++ 6.4.5 Settings->Preferences->General->Document List Panel-> check "Show"
I found Document Switcher in General tab in prefs. Then I clicked the Show checkbox and that did the trick.
Make sure you have the latest Notepad++ version.