What can kill someone with a very small amount?

So, this is from my own bored, strange mind when left to wander. Don't worry, I wouldn't harm any one else with it. I wouldn't harm a fly! I was just wondering what substance (when ingested) can kill a person very quickly (within a day or quicker) with a very small amount (like, the size of a pill). Any ideas? Also, how does it kill them, how painful would it be, would they be concious for the entire (or a good chunk) of the time?