(Please note, this was written for a site pertaining to dogs and is currently waiting to be updated with hazards for other types of animals.)

This is not a complete list of all things toxic to your canine friends but it does contain most of the more common threats. As with anything you do, use your common sense and judgement to keep potentially harmful things away from your dog, and if your dog SHOULD get to something that you don't know is safe, get him to the vet as soon as possible. If you know what your dog consumed, you can alternatively contact the ASPCA Poison Control hotline (may be limited to US only, I don't know) for a fee, at 888-426-4435.

Symptoms of poisoning are not limited to but include vomiting, lost appetite, uncoordinated movements (staggering, stumbling around, unable to stand perhaps), lethargy, pain, shock, seizures, abnormal breathing, drooling/foaming at the mouth... the list goes on. Some poisons are fast-acting and you will be rushing to the doctor's office right away, but others can take days or even a week or so to actually cause distress. You should watch your dog for signs of toxicity for at least a couple weeks after ingesting the poison.

Symptoms of obstruction include vomiting or dry-heaving, lost appetite, drooling, loose stool or tenesmus (struggling to eliminate), bloating or abdominal pain, lethargy.

Common Hazards include:

FOOD:

• Chocolate- In addition to caffeine, chocolate contains a toxic chemical called theobromine. The less sweet- or, the darker- the chocolate, the more potent. Six squares of baking chocolate per 20 lbs is considered lethal for canines. Milk chocolate is much less dangerous but should not be given to dogs.
• Raisins- Raisins (and the grapes from which they are derived) are also toxic to dogs, causing acute renal failure, though the actual cause is unknown. Large quantities are generally required to cause a reaction but sensitive dogs can be affected more easily.
• Fatty foods such as poultry trimmings, gravy, etc can cause GI upset and should be avoided. Cooked turkey and chicken bones can splinter. Actually, it's best to toss leftovers rather than give them to your pets. Too much people food can give diarrhea. If you're tossing those leftovers (or even aluminum foil from cooking or something like that) in the trash can, secure it so they don't rummage for leftovers themselves!
• Poultry and other bones (cooked)- may splinter, cause a choking hazard or damage to the esophagus/intestines if swallowed.
• Onions in any form, or garlic (large quantities only)- contain sulfides and can cause hemolytic anemia.
• Nuts- macadamias are commonly found in cookies and other foods around the holidays. While no nut is totally healthy for our pets, macadamias- as few as 6 shelled nuts- can cause a loss of motor skills, development of tumors, and other ill effects.
• Alcohol- just an ounce can put a small dog or cat into a coma!
• Coffee, soda- caffeine is a big no-no for your dog, and coffee beans should NOT be eaten
• Fried foods- grease and oil used to fry foods or that which is extrapolated from fatty meats, etc, can cause immediate effects like upset stomach and often diarrhea. Long term feeding (ie, a dog who eats fatty/greasy table food nearly every day) can result in pancreatitis.
• Potatoes- the "eyes" and any green spots under the skin of potatoes contain a toxin called solanine, which is harmful to many animals including dogs (and people, btw!) Dogs can eat potatoes but make sure there are no sprouts or green patches. Fried potatoes contain a lot of grease and usually sodium, and so should be given in moderation only if at all.
• Tomatoes- the leaves and stems from tomato plants contain a high level of atropine, another toxin. Tomatoes themselves can also have this chemical but in smaller amounts. The fruit itself contains tomatine, which is similar to the solanine found in potatoes and you guessed it- another toxin. The riper the tomato, the less atropine or tomatine, but it's best to avoid the feeding of tomatoes to avoid a reaction. Tomatoes contain a high level of citric acid which can cause stomach upset and indigestion.

• Household cleaners and chemicals- unless actually stated on the bottle that it is a non-toxic, water based solution that can be ingested safely, assume that all your cleaners are unsafe. Keep your pets off freshly mopped floors or dry the floors with towels, and rinse his feet immediately if your dog or cat walks on them to avoid ingestion through paw-cleaning later!
• Health Supplies- Even medication meant FOR your dog or cat may be harmful if ingested (creams/ointments) or if ingested in large quantities (overdose, pills). Many medications for humans are not safe for dogs (aspirin, ibuprofen, cold medicine) and even vitamins in excess dosages can cause toxic reaction. Dogs may find tubes of hand cream, ointment, etc fun to chew and accidentally ingest the contents. Just as with a child, keep them away, safe in a cabinet and out of reach.
• Toothpaste/Dental floss: fluoride is toxic to dogs, and swallowed dental floss can cause intestinal blockage or strangulation. Use only toothpaste made for dogs when brushing their teeth.
• Over-the-counter flea/tick medications- use ONLY products for fleas and ticks that have been prescribed by your veterinarian. Many dogs and cats have become ill or died when given OTC drugs like Hartz, Zodiac, and even Biospot. Pyrethrin and Permethrin are toxic insecticides that work their way into your dog's bloodstream- do NOT use any product listing them as an ingredient.
• Poisonous plants- mistletoe, rhubarb, diffenbachia, azalea, lilies, to name a few. See the links below for more:
  
  http://www.hsus.org/pets/pet_care/protect_your_pet_from_common_household_dangers/common_poisonous_plants.html
  
  http://www.cybercanine.com/toxicplants.htm

• Antifreeze- ethylene glycol tastes sweet and attracts dogs and cats but will kill if swallowed in even small amounts. Check your vehicle for leaks, don't allow your cats or dogs to roam free where they can contact antifreeze puddles, and consider using a propylene glycol-based antifreeze that is safer if ingested (though still toxic in larger doses.)
• De-icers- the "salt" used to melt ice can irritate your dog's feet and contains toxins that your dog will lick off while cleaning his feet. Clean your dog's feet after bringing him in for a walk in the winter, try doggy boots that protect his precious pads, and use a dog-safe de-icer around your home.
• Lawn chemicals/plant food- Can cause allergic reactions/irritation when coming in contact with the skin, and can be ingested when your dog licks it off his feet/legs/etc. Don't use lawn chemicals/fertilizer/chemical plant food in the yard where your dog walks.
• Mulch- some types of mulch especially cocoa mulch can be deadly if swallowed.
• Leaves- oak, red maple, locust, and several other trees have leaves which can be poisonous if eaten. Some trees also have toxic bark. Check the poisonous plants lists posted above if you have a dog who likes to chew on sticks/bark/leaves in your yard.

• String/rubber bands/twine/yarn, etc etc etc- Any of these things, like dental floss mentioned above, can wrap around things in the stomach and create a blockage or wrap around part of the intestinal tract and strangle the tissue.
• Toys- baby and children's toys sometimes have small parts like eyes or noses, strings, or other small pieces that can be a choking hazard. Some are stuffed with poly "beans" instead of fiber fill. Even certain dog toys have small parts that could be chewed off and swallowed or squeakers that can be ripped out, and "rope" toys made of braided threads can be shredded and the string ingested. Never allow your dog to have a toy unsupervised that has any loose part, strings, or any other danger. Instead, if leaving your dog alone use a heavy chew toy like a Kong to keep him busy.
• Chew toys/bones- Never leave your dog unattended with raw bones, rawhide, or any other "treat" of that kind. They are known choking hazards and should only be given under direct supervision.
• Power cords- this goes without saying!
• Coins, batteries- both can cause obstruction of the intestinal tract. Batteries, if punctured by the dog, contains potassium hydroxide which is a corrosive agent that can burn the inside of the mouth, esophagus, and stomach