Gravity Gun. In reality, a portal gun would be dangerous due to lack of Long-Fall Boots, and impractical because portals can only be opened on certain surfaces - concrete, lunar paste.
The financial applications are almost endless, namely hiring it out for heavy lifting or toxic substance manipulation.
Then again, if the Portal Gun works the same as it does in Portal 2 and can be shot to the moon, this has exciting applications for space exploration. Not having to expend logistics and spacecraft to GET to the moon would be a huge leg-up.