This is a very simple one, but something that is quite useful.
On Mac, when you right-click on a particular word, an option in the context menu that pops up should be: Look Up In Dictionary.
Clicking on that should fire up the Mac dictionary and display the selected word.
Half of this issue is already implemented... The integration with dictionary is already implemented by OSX. select a word, then click on Sublime Text 2 (in the menu on the top) > Services > Look Up in Dictionary.
All that needs to be done is to add an item in the right-click context menu that fires Look Up in Dictionary.
I personally use it a lot and is very handy. I know I can setup a keyboard shortcut to it, but it'd be good to have it in the context menu as well.