Guys,
Please don't dump it, or pour it on the grass.
And definitely do not leave it in the open (open bucket, pooled on the ground, etc.), as most coolants are quite attractive and toxic to animals. Including humans (ahem, kids), wildlife, salmon, and pets I have already had 2 dogs poisoned by folks being ignorant (these were not from coolant); one survived, one died a gut-wrenching death.
Collect it and dispose of it per your local gov regs/suggestions. Use the EPA site noted above and the web to find local disposal sites. Most municipal wastewater treatment systems are not designed for properly treating wastes like this (I worked on wastewater systems and associated contaminants for 3 years; these plants are designed for turds and piss), but IMO it is probably a bit better to put it down the (non-septic tank) sewer than on your lawn, and much better the sewer than the storm drain (gutter). Proper disposal facility is by far the best. If you spill a small-moderate amount, then use an old towel and soak it all up (then trash the towel). Have those towels handy when you do your work. If you are going to dilute what you spilled, you probably should very heavily dilute it.
One poster stated that most modern coolants are propylene glycol (and ... harmless).
Well, Toyota Red and Pink are ethylene glycol, which is far from harmless. Prestone Dexcool is ethylene glycol. Prestone extended life is ethylene glycol. Amsoil and Sierra anti-freeze are propylene glycol. Most of us are likely using the toxic stuff.
I provided the MSDS sheet links below.
Toxins (Poisons) That Effect Dogs - VetInfo
The most commonly used antifreeze compounds contain ethylene glycol, which is toxic to dogs and cats if ingested. The minimal lethal dosage for cats is 1.4ml/kg of body weight and for dogs 4.4 to 6.6 ml/kg of undiluted ethylene glycol, according to Thrall, Grauer and Dial in Kirk's Current Therapy XII. This means that 1 teaspoonful of antifreeze (5ml) could kill a 7 pound cat. Antifreeze is normally diluted for use in automobiles so more of the diluted solution would have to be ingested to achieve toxic levels. Unfortunately, antifreeze seems to be palatable to dogs and cats and ingestion of large amounts of the solution can occur.
Here are the MSDS sheets:
http://www.worldpac.com/tagged/Antifreeze-Coolant_002721LLAC.pdf
WORLDPAC - Material Safety Data Sheets
http://www.sciencelab.com/msds.php?msdsId=9927239
http://www.amsoil.com/msds/ant.pdf
Thanks, Jon