Is NO2+ Polar or Nonpolar?

Answer: NO2+ is a nonpolar molecule because of its linear structure which cancels out opposing dipole forces. This results in a molecule that does not have permanent partial charges. 

This is also called the Nitronium ion not to be confused with the nitrite ion (or NO2-) which does not have a linear structure. It is a cation similar to ammonium (NH4+) in regards to its charge. This +1 means that the molecule has a high affinity for electrons. In fact, there have been recorded instances where the molecule added in an extra electron to produce a stable compound. While NO2+ can be produced and thereafter exist stably in normal conditions, it is quite reactive on its own merit. Like NH4+, nitronium can form a few different types of salts and molecular compounds where the positive cation is balanced out by a very electronegative or anion species that thereby balances charge across the entire molecule.

NO2+ Ball and Stick Model. Created with Avagadro.

What is the most common usage of NO2+?

Within industrial and laboratory settings NO2+ is most like to be utilized in the nitration of other substances (i.e. the addition of nitronium to a molecule). For example, this article goes through the necessary chemical reactions in order to add NO2 to a benzene ring: https://www.chemguide.co.uk/mechanisms/elsub/nitrationtt.html