Grenades first appeared in Postal, and can be found hidden in various areas of levels, and being used by Grenade throwers. The longer the fire key is held in, the further Postal Dude will throw the grenade. After a couple of seconds the grenade will explode, killing an enemy in the vicinity.
"The Grenade can be used to create small explosions. The longer you hold down the fire key before releasing it, the further the Grenade will be thrown. This weapon also has a secondary fire option, allowing you to place them down safely without triggering the fuse. Then, you may fire at them with the pistol or other weapons from a safe distance to detonate them. Walk back over them to simply pick them back up. For other people around you though, the unarmed grenade will act like an anti-personnel mine and detonate on contact. This is great for setting up traps! Not that this is recommended or encouraged, but hey, they call it “free will” for a reason." — The grenade's description in the manual
Grenades reappear in Postal 2 and its expansions. If used effectively, they can provide great amounts of damage. They are generally quite hard to use, as time, angle and pitch comes into account, however if a grenade hits someone it will instantly explode. The secondary fire mode for grenades makes Postal Dude pull the pin and set one down, thus detonating when stood on by another person like a mine. Grenades thrown by an enemy can be kicked back to the one throwing them, if it's done quickly and with accuracy. Grenades deal about 40 damage.
Grenades are quite rare, but are mostly found in the hands of Terrorists or in secret areas. Postal Dude will also always pull out a grenade and put it in his mouth when committing suicide, using a yellow Grenade in Enhanced Mode which makes a gigantic explosion similar to those of the Mini-Nuke Launcher.
Grenades function similar to how they did in POSTAL 2 but are easier to aim with the fire button held down to aim and alt fire to throw. Just hitting the Alt fire will place them on the ground as mines.