The grenade of the Covenant. Like most of their weapons, a relatively standard device with one or two clever twists.
The most important - and most ingenious - is that plasma grenades will stick. Throw them at an enemy - any enemy - a vehicle, or even just at the ground, and it will stick on and stay there. This has a number of applications. The most practical is that, when fighting a difficult-to-tackle vehicle, all you've got to do is stick a p-grenade on it, and it'll blow, no matter how fast it tries to run. The most fun is slapping a p-grenade to your friend in multiplayer or to a Grunt in single-player - all they can do is stare at it, until - blam - up they go (beware, however, of kamikazes attempting to take you with them). More practically, a nasty enemy like an Elite can be disposed of with a single 'nade. And an interesting tactical consideration is that plasma grenades will stick to the ground (though not to a wall or ceiling) instead of bouncing - so if you're trying to lob one onto a ledge or into some miniscule buttress-like outcropping somewhere, go for a plasma instead of a frag.
Plasma grenades are so common that it's almost worrisome. Take a stroll after a nasty battle against a group of Covenant, and you'll see so many that you'll think they're sprouting from the ground. It's never a problem to max yourself out with them - so be prolific in their use. However - there is one thing to keep in mind. Those numerous grenades on the ground, scattered like so many baseballs, are not stable. Apparently, the human scientists did one thing right: fragmentation grenades won't explode off-hand (well, they can... but they require special circumstances, and usually won't.) But plasma grenades? Well, a single well-placed explosion can touch them off easily, and that field of grenades could well turn into a minefield. You can use this to your advantage - but it can also screw you over, so be careful, lest a single grenade-flipping Grunt bring about your demise.
P-grenades have a three-second fuse, activated once they stick or come to rest. Three seconds, unfortunately, can be an eternity in fast-paced combat, so plan accordingly. And one note: plasma grenades come equipped with a sort of blue "flare" effect, making them highly visible indeed. If you're trying to send someone (or a bunch of someones) flying without making them run from you, use a frag - they'll notice a plasma.
And a final awareness-raiser: like frags, p-grenades won't take out a tank in one hit. Be warned.
Pros: "Sticky" behavior, high damage, salvageable abundance, instability.
Cons: "Sticky" behavior, instability.
Usage: Got talent? Slap one on an enemy (or one of your friends in multi), sending him up in shards. Don't got talent? Use it like an ordinary grenade - it'll be easy to aim, since it won't roll. Remember them when confronted by vehicles.