REQUIRES CS:S or at least the hands and stock knife. You might find a replacement at gamebanana or someplace if you looked.

So what we have here is a remotely detonated improvised explosive device. Disguised as an inconspicuous box and detonated by cell phone, you can place multiple boxes and detonate them all at once.

Press Fire to drop an IED, press alt fire to detonate. Uses regular grenades for ammo, and spawns with just one. So give yourself more if you want to detonate multiple bombs.

Credit to Garry/valve for the box and phone, and Failure for the particle effects.