Antifreeze is a common poison. It has a sweet taste and cats will consume it readily. As little as one teaspoonful can be lethal for a 7 pound cat.
Antifreeze poisoning causes nervous systems signs and severe kidney failure. It can be fatal if not treated very soon after ingestion (within 4 to 8 hours). After ingestion, cats may act "drunk" within 30 minutes to 12 hours, depending on the amount ingested. Many times you don't notice these signs after which your cat can act normal as internal organs begin to fail.
For more information, please read the article Ethylene Glycol Toxicosis.