Apart from weapons, other inventory objects also provide extra skills. Specifically,you have a utility belt that can hold grenades. 

To throw a grenade, press and hold 3 on the keyboard and aim with your mouse. Release the key to throw.

The longer you keep the button pressed,  the higher the arc of the throw will be. A high arc is useful to lob grenades over taller objects, but of course the grenade will spend more time in the air, making it somewhat more difficult to hit enemies.

Note that unlike weapon skills, grenades don’t have unlimited uses. You start out with a limited supply and you can gain extra charges from Supply boxes during the mission.
​