A grenade that acts like the electro-balls from Half-life 2. You throw it and it reacts to enemy vehicles and units, chases them then electracutes or EMPs anything near it. It can't be blown up, because it shocks/scrambles any ordinence fired at it, it can't be picked up because obviously it would electracute you, and because it's technicly not an explosive device it could potentially last for days, and when it does eventially run out of juice or is destroyed it would just act like a conventional frag and shoot off bits of itself everywhere.
Not only that but you could program it for different tasks, for instance an anti-personel version for throwing into a room that shocks and blinds anyone inside that isn't friendly before storming a room. Or heavier versions for bombing runs, like a MOAB version, or cluster-bomb varients that send small ones rolling all over an enemy position.
I really did miss my true calling.