Mac Dictionary improvements
Try ^+⌘+D while selecting a word (that's ctrl+cmd+d). If the app is well integrated into Mac OS X, it will show a little popup with the word definition.
Like so
If you click "More..", it'll open the dictionary app.
If this shortcut doesn't work, go to
System Preferences > Keyboard > Keyboard shortcuts > Services
and search Look Up in Dictionary to add the shortcut you want.