Are there any keyboard shortcut prompt applications?
While this is not a solution you're asking for, you can try adding shortcut in System Preferences -> Keyboard -> Keyboard Shortcuts -> Application Shortcuts for menu item 'Show Help Menu' (I'm not sure if it's enabled by default).

Then after pressing this shortcut you can type command name and help system will search for that in menu. I guess names of actions fairly often - Close Tab is quite easy to guess, as you just need to start typing 'Close'.

I hope this will help.