Institute Portal Gun
If you haven't tried the Institute Portal Gun yet, make sure you give it a try. It's fun to use and it'll provide you with some hilarious moments, guaranteed.
The Institute Portal Gun shoots, you guessed it, portals. One is blue, one is orange. (If you've played the Portal games this probably sounds familiar.)
The portals allow you to get to areas like the grandstands in Diamond City or to the top of the water towers located around the Commonwealth. You can also use it to quickly escape battles or quickly steal items when the owners aren't looking.
Tons of possiblities, tons of fun, highly recommended.