The strangler

The strangler is a rare and extremely deadly poison, which kills by making a person who has ingested it unable to breathe.

It causes the throat to swell shut, and to the untrained eye its effects may be confused with a severe choking fit, though it also causes blood to leak out of bodily orifices such as the mouth, nose, and eyes. The effects are almost instantaneous after being ingested.

The poison takes the form of inert crystals which can be crushed and dissolved into beverages for the targeted victim to drink.

Known victims

 * Maester Cressen attempted to assassinate Melisandre by offering her a cup of wine containing the strangler. As an old man and more concerned about Stannis's well-being, Cressen knowingly drank from the poisoned cup as a sign of good faith, in the hope that this would trick the Red Priestess into accepting his offer.  To Cressen's surprise, Melisandre's magical abilities apparently protected her from the effects of the poison, while Cressen himself collapsed and died.
 * King Joffrey Baratheon was assassinated using the strangler, smuggled into his wedding disguised as crystals in a necklace worn by Sansa Stark.

In the books
The poison is always referred to as "the strangler" with a lowercase "s".

The TV series did not explain it at the time, but the poison used to kill Joffrey is the same kind that Maester Cressen used two seasons earlier in an attempt to kill Melisandre. Half a swallow of wine spiked with the strangler was sufficient to kill Crssen, while Melisandre finished almost an entire cup of poisoned wine and was unaffected, apparently due to her magical powers.