LD50 means 'Lethal dose -50% kill rate" to put it simply. It means its the dose at which minimum 50% of test subjects are killed. Thats it in a nutshell.
Here is a list of LD50's I ripped off WIKI
Substance Animal, Route LD50 Reference
Water rat, oral 90 mL/kg (~90,000 mg/kg) [7]
Sucrose (table sugar) rat, oral 29,700 mg/kg [8]
Vitamin C (ascorbic acid) rat, oral 11,900 mg/kg [9]
Grain alcohol (ethanol) rat, oral 7,060 mg/kg [10]
Table Salt rat, oral 3,000 mg/kg [11]
Paracetamol (acetaminophen) rat, oral 1,944 mg/kg [12]
THC (main psychoactive substance in Cannabis) rat, oral 1,270 mg/kg males; 730 mg/kg females [13]
Aspirin (acetylsalicylic acid) rat, oral 200 mg/kg [14]
Caffeine rat, oral 192 mg/kg [15]
Nicotine rat, oral 50 mg/kg [16]
Strychnine rat, oral 16 mg/kg [17]
Sodium cyanide rat, oral 6.4 mg/kg [18]
Aflatoxin B1 (from Aspergillus flavus) rat, oral 0.048 mg/kg [19]
Dioxin (TCDD) rat, oral 0.020 mg/kg [20]
Batrachotoxin (from poison dart frog) human, sub-cutaneous injection 0.002-0.007 mg/kg (estimated) [21]
Polonium 210 human, inhalation 0.00001 mg/kg (estimated) [22]
Botulinum toxin (Botox) human, oral, injection 0.000001 mg/kg (estimated) [23]
Hydrocodone: 375 mg/kg
Alcohol: 10.6 g/kg
THC: 1270 mg/kg (No one has ever OD'd on weed)
Nicotine: 50 mg/kg
Vitamin C: 11.9 g/kg
Caffeine: 192 mg/Kg
Cocaine: 95.1 mg/kg
DMT: 110 mg/kg
5-MeO-DMT: 115 mg/kg
MDMA: 350 mg/kg
Codeine: 427 mg/kg
Mescaline: 880 mg/kg
Diphenhydramine: 500mg/kg
Morphine: 461 mg/kg
Alprazolam: 331 mg/kg
Phenobarbital: 176 mg/kg
Diazepam: 720 mg/kg
Methqualone: 326 mg/kg
Psilocybin: 280 mg/kg
Methamphetamine: 55 mg/kg
Methylphenidate: 350 mg/kg
Zolpidem: 695 mg/kg
Aspirin: 1500 mg/kg
Acetaminophen: 500 mg/kg
Fentanyl: 0.03mg/kg
Ketamine: 500 mg/Kg
GHB: 1100mg/kg
Dextroamphetamine: 96.8 mg/kg
Methadone: 30 mg/kg
PCP: 130 mg/kg
Codeine: 427 mg/kg
Mescaline: 880 mg/kg
Diphenhydramine: 500mg/kg
Morphine: 461 mg/kg
Alprazolam: 331 mg/kg
Phenobarbital: 176 mg/kg
Diazepam: 720 mg/kg
Methqualone: 326 mg/kg
Psilocybin: 280 mg/kg
Methamphetamine: 55 mg/kg
Methylphenidate: 350 mg/kg
Zolpidem: 695 mg/kg
Aspirin: 1500 mg/kg
Acetaminophen: 500 mg/kg
Fentanyl: 0.03mg/kg
Ketamine: 500 mg/Kg
GHB: 1100mg/kg
Dextroamphetamine: 96.8 mg/kg
Methadone: 30 mg/kg
Diphenhydramine: 500mg/kg
LSD : 12,000 ug => 12mg/kg
Clonazepam: > 4000 mg/kg
Iron (often used as a nutritional supplement): 1250 mg/kg
Ambien: limited info, anecdotal report of recovery from ingestion of 400 mg
Links...
http://www.asanltr.com/newsletter/02-2/articles/Neurotoxins.htm
http://www.erowid.org/psychoactives/health/psychoactives_ld50s.shtml
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/L/LD50.html
http://paipm.cas.psu.edu/1024.htm
http://delloyd.50megs.com/hazard/LD50.html
http://web.singnet.com.sg/~chuaeecc/venom/rpotent.htm
http://www.bnl.gov/esh/shsd/Programs/Program_Area_Chemicals_Highly_Acute_Toxins.asp
http://www.ilo.org/public/english/protection/safework/cis/products/safetytm/toxic.htm
http://mv.lycaeum.org/mu/LD50_list.html
More once I have time. Sorry for duplicates in the list - it was compiled from many sources.
Here is a list of LD50's I ripped off WIKI
Substance Animal, Route LD50 Reference
Water rat, oral 90 mL/kg (~90,000 mg/kg) [7]
Sucrose (table sugar) rat, oral 29,700 mg/kg [8]
Vitamin C (ascorbic acid) rat, oral 11,900 mg/kg [9]
Grain alcohol (ethanol) rat, oral 7,060 mg/kg [10]
Table Salt rat, oral 3,000 mg/kg [11]
Paracetamol (acetaminophen) rat, oral 1,944 mg/kg [12]
THC (main psychoactive substance in Cannabis) rat, oral 1,270 mg/kg males; 730 mg/kg females [13]
Aspirin (acetylsalicylic acid) rat, oral 200 mg/kg [14]
Caffeine rat, oral 192 mg/kg [15]
Nicotine rat, oral 50 mg/kg [16]
Strychnine rat, oral 16 mg/kg [17]
Sodium cyanide rat, oral 6.4 mg/kg [18]
Aflatoxin B1 (from Aspergillus flavus) rat, oral 0.048 mg/kg [19]
Dioxin (TCDD) rat, oral 0.020 mg/kg [20]
Batrachotoxin (from poison dart frog) human, sub-cutaneous injection 0.002-0.007 mg/kg (estimated) [21]
Polonium 210 human, inhalation 0.00001 mg/kg (estimated) [22]
Botulinum toxin (Botox) human, oral, injection 0.000001 mg/kg (estimated) [23]
Hydrocodone: 375 mg/kg
Alcohol: 10.6 g/kg
THC: 1270 mg/kg (No one has ever OD'd on weed)
Nicotine: 50 mg/kg
Vitamin C: 11.9 g/kg
Caffeine: 192 mg/Kg
Cocaine: 95.1 mg/kg
DMT: 110 mg/kg
5-MeO-DMT: 115 mg/kg
MDMA: 350 mg/kg
Codeine: 427 mg/kg
Mescaline: 880 mg/kg
Diphenhydramine: 500mg/kg
Morphine: 461 mg/kg
Alprazolam: 331 mg/kg
Phenobarbital: 176 mg/kg
Diazepam: 720 mg/kg
Methqualone: 326 mg/kg
Psilocybin: 280 mg/kg
Methamphetamine: 55 mg/kg
Methylphenidate: 350 mg/kg
Zolpidem: 695 mg/kg
Aspirin: 1500 mg/kg
Acetaminophen: 500 mg/kg
Fentanyl: 0.03mg/kg
Ketamine: 500 mg/Kg
GHB: 1100mg/kg
Dextroamphetamine: 96.8 mg/kg
Methadone: 30 mg/kg
PCP: 130 mg/kg
Codeine: 427 mg/kg
Mescaline: 880 mg/kg
Diphenhydramine: 500mg/kg
Morphine: 461 mg/kg
Alprazolam: 331 mg/kg
Phenobarbital: 176 mg/kg
Diazepam: 720 mg/kg
Methqualone: 326 mg/kg
Psilocybin: 280 mg/kg
Methamphetamine: 55 mg/kg
Methylphenidate: 350 mg/kg
Zolpidem: 695 mg/kg
Aspirin: 1500 mg/kg
Acetaminophen: 500 mg/kg
Fentanyl: 0.03mg/kg
Ketamine: 500 mg/Kg
GHB: 1100mg/kg
Dextroamphetamine: 96.8 mg/kg
Methadone: 30 mg/kg
Diphenhydramine: 500mg/kg
LSD : 12,000 ug => 12mg/kg
Clonazepam: > 4000 mg/kg
Iron (often used as a nutritional supplement): 1250 mg/kg
Ambien: limited info, anecdotal report of recovery from ingestion of 400 mg
Links...
http://www.asanltr.com/newsletter/02-2/articles/Neurotoxins.htm
http://www.erowid.org/psychoactives/health/psychoactives_ld50s.shtml
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/L/LD50.html
http://paipm.cas.psu.edu/1024.htm
http://delloyd.50megs.com/hazard/LD50.html
http://web.singnet.com.sg/~chuaeecc/venom/rpotent.htm
http://www.bnl.gov/esh/shsd/Programs/Program_Area_Chemicals_Highly_Acute_Toxins.asp
http://www.ilo.org/public/english/protection/safework/cis/products/safetytm/toxic.htm
http://mv.lycaeum.org/mu/LD50_list.html
More once I have time. Sorry for duplicates in the list - it was compiled from many sources.