LD50

Substance (source or product) LD50(mouse or rat)
Botulinum toxin(bacteria)* 0.3 ng/kg
Ricin*(from caster bean plant) 0.001 mg/kg(man)
Batrachotoxin*(from poisonous frog) 0.002 mg/kg
Vx (nerve gas) 2 x 10-2 mg/kg
tetrodotoxin*(poison from puffer fish) 0.334 mg/kg
Nicotine* 2 mg/kg
benzaldehyde*(Cherry Flavor) 4.8 mg/kg
parathion(insecticide) 6.0 mg/kg
Sodium cyanide 10 mg/kg
Cocaine* 17.5 mg/kg
Lead 20 mg/kg
amphetamine sulfate 32. mg/kg
sodium fluoride(toothpaste) 52. mg/kg
Tetrahydrocannabinol* (THC form marijuana)* 110. mg/kg
diazinon(ant killer dust) 76 mg/kg
Caffeine* 200 mg/kg
Caffeine*(Mountain Dew) 130 mg/kg
Heroin 150 mg/kg
Nicotine*(through mouth) 190. mg/kg
potassium nitrate(fertilizer) 190. mg/kg
Morphine* 500 mg/kg
ephedrine* 0.60 g/kg
Ethanol 1.0 g/kg
malic acid*(sour candy) 1.6 g/kg
Aspirin 1.75 g/kg
disodium EDTA(Secret) 2.00 g/kg
Ethyl acetate*(Cherry Flavor) 6.10 g/kg
Methanol(wood alcohol) 5.628 g/kg
propylene glycol(Cherry Flavor) 20. g/kg

References
1. Arnon, Stephen. "Botulinum Toxin as a Biological Weapon." JAMA. vol 285. pp.1059-1070. 2001
2. Gill MD. Bacterial toxins: a table of lethal amounts. Microbiol Rev. 1982;46:86-94. Full text

* natural substances

Due to species differences in LD50 values, defining risk to human beings based on animal is difficult, and may be a lot diffirent.
For example, LD50 values for dioxin(2,3,7,8-TCDD)
species LD50(mg/kg)
Guinea pig 0.0006
Rat 0.04
Monkey 0.07
Rabbit 0.115
Dog 0.150
Mouse 0.200
Hampster 3.5
Bulldog > 1.0

see polychlorinated_dibenzodioxins

Some LD50 values for average humans are
substances LD50(mg/kg)
Morphine 1-50
Aspirin 50-500
Methanol 500-5000
Ethanol 5000-15000