What is the keyboard shortcut for "Open With" in Finder?
The best solution for me: 


Select file you want to open
Press ⇧+⌘+/ (opens up the mighty Help search)
Then start to type a name of app you want to open
Use arrow keys to select right menu item
Press Enter
Done! 


For example I want to open HTML-file with Sublime Text instead of Chrome. 
I navigate to this file, press shortcut, type in "su", press down-arrow and press enter!