Drop osmium nails from airplanes!
Osmium is the densest (substance, or merely element?) known. A brick of osmium weighs about fifty pounds. Logically, osmium nails could weigh maybe a pound each. And they would have three effects.
1) Impact damage. Something nail-shaped weighing a pound would have an absurd terminal velocity. They'd hit ground traveling incredibly fast - that's a lot of momentum. Imagine what this would do to any sort of soft target. Or even mildly armored targets. Tip them with something that holds an edge (I dunno if osmium does) and you've got armor-piercing. No gunpowder is necessary, they'll take their speed from gravity. Ow.
2) Poison. Osmium, being a heavy metal, is poisonous to humans.
3) Toxic gas, which osmium gives off in contact with air. Admittedly you'd end up with multi-mile-high towers of toxic gas where you dropped them, but I'm more thinking about the impact point. Especially if they shatter - does osmium shatter? You might make the entire area uninhabitable until it stops spewing toxic gas . . . trying to clean up the osmium would be nasty, you'd basically have to scrape off a few inches of soil and throw it away.
Of course, it's totally impractical, but still ;)