One I like is Ctrl+Cmd+d while the pointer is over a word will pop up a small dictionary with a definition of that word. This does not work in every application

One I like is Ctrl+Cmd+d while the pointer is over a word will pop up a small dictionary with a definition of that word. This does not work in every application (e.g. it doesn't work in Chrome or Firefox), but it should work in any Cocoa based application.